Engine Knocking Noise While Idling

Hello,

"For some time now, my car has been making a combination of clicking and rumbling noises when coasting, specifically when there is no power transmission between the engine and the wheels. This happens in neutral, with a gear engaged and the clutch depressed, or when the car is rolling in first gear at idle speed on a slight downhill slope, where the transmission is alternately experiencing pushing and pulling forces."

It rumbles as long as the vehicle is moving forward or backward, even at a very slow pace.
It's only when the car is completely still that there is definitely silence. If you release the brakes on a slope, the roaring continues as soon as the wheels begin to move.

In addition, the device emits a noise, similar to that of a random generator, which is irregularly turned on and off, unfortunately with a noticeable bias towards the "on" state.
According to this, it is not a normal material resonance that oscillates when properly stimulated and then remains constant.

It occurs a bit sooner and louder when the machine is already warm compared to immediately after a cold start.
Applying a little gas up to around 1200 rpm has no effect on the sound; at higher speeds, it is drowned out.

The sound seems somehow related to the clicking noise that comes from the transmission or dual-mass flywheel when the engine speed doesn't match the gear being engaged, or when you push the car forward or backward while it's in gear.
If I imagine that clicking sound happening rapidly in succession, it could potentially create a rumbling noise as it unfolds.

So, I moved the car forward a few meters with the engine running and the hood open, in order to locate the noise coming from the engine compartment.
But I didn't even hear it: under these conditions, it will probably be drowned out by the engine .
Well, it's probably not a very serious issue, because expensive sounds usually have a brighter frequency spectrum, which remains noticeable even with loud background noise... I still have almost 2 years of warranty, but you usually have to tell the workshops what to do anyway - otherwise, you'll probably have to take the car in for free repairs five times (and have it damaged accordingly ).

When it comes to the topic of ZMS noise, the enlightened driver of a Porsche naturally thinks of the anti-quiver system. However, my car has only rarely and mildly exhibited the characteristic engine vibrations, while the rumbling sound, under the aforementioned conditions, practically always occurs.
However, the values of the idle speed control do not noticeably change when the engine starts to roll over (with a rumble) or when it comes to a stop.

Is there anyone else with similar experiences?
Who has an idea what the cause might be?

Hello Ulf, the noise likely comes from the transmission and is referred to as 'clattering of the clutch release bearing.'

mersente wrote:

Hello Ulf, the noise probably comes from the transmission and is referred to as clutch rattle.

Yes, I had also been thinking about something like that... but why can it (so far) be switched off so reliably by stopping (the differential) ?

Hi Ulf,

Please check if the factory-filled transmission oil is the correct amount.
They probably cut corners on my A3.

Best regards, Rainer.

Rainer K. wrote:

Please check if the factory-filled transmission oil is also the intended fill quantity...

Hi Rainer,

I wanted to get that done today at a new Opel service center (an old Opel dealership that now also services VW cars and has hired some VW-trained mechanics).

Result 1: ELSA (a vehicle repair manual) provides no information regarding checking the transmission oil level for my car (GKB = FSM), so it doesn't indicate where the dipstick is located, or even whether there is one at all.

Result 2: AKTE indicates a "closure screw" (M 24) at a plausible location.
Okay, so I lifted the Polo, removed the oil pan, and examined the screw: basically, it's similar to the mass airflow sensor screws (which are Torx screws), with a size roughly equivalent to a "sausage finger," and it has a multi-tooth design.

The master didn't even know if he had anything like that exotic equipment in the workshop, and since there was no mention of the necessary special tool in the work instructions, nothing could be done.
Maybe sometime soon... so for now, I'm going to keep going and let it all out.
However, he offered to carefully document my complaint during a new appointment and report it to the service department, so that I might still be eligible for a replacement gearbox even after the warranty period expires – I found that acceptable.

Regarding the wheel rattle / @mersente:
It cannot be impressed by simply engaging or disengaging.
That means that even if the transmission input is stationary and the differential is rotating at, say, 4 rpm (meaning the vehicle is moving at a snail's pace), it still makes a roaring noise.
-> Even a small amount of movement in the transmission can still cause a rattling noise from the freewheel that is clearly audible (inside the vehicle).

Alternative approach:

Did you use different wheel bolts for your new rims? If so, were they longer? If yes, then your wheel bolts might be slightly rubbing against the inner part of the wheel bearing housing.

If no... Transmission oil refill. There is no dipstick; the bottom edge of the oil filler screw is usually the reference point.

Julian wrote:

Did you use different wheel bolts for your new rims?

Hi Julian,

Well thought out , but a wrong lead -> both are steel rims with the same hub and are attached using the original bolts.

Quote:

If no....Transmission oil refill. There is no dipstick; the bottom edge of the oil filler screw is usually the reference point.

I also plan to check it - but where can you get such a tool ?
Have you had similar experiences with noises that were caused by insufficient oil?

Hmm, I suppose the steering wheel is also secured with something similar, right? There's a 12-tooth spline on the steering wheel.

Best regards, WarLord.

WarLord wrote:

Hmm, the steering wheel is probably also secured with something similar, right? There's a 12-tooth spline on the steering wheel.

Hi,

After that, I haven't looked - but I saw so many small bolts on the chassis on stage yesterday - and that can only mean that VW wants to make it as expensive as possible for DIYers .

(But by foregoing the climate control and other common features, I still have enough money saved from buying the Polo to be able to afford all the necessary spur gears in gold, if necessary .)


Back to the topic:
Today, I wanted to disconnect the speed sensor connector and see if the rumbling noise might be influenced by different idle control strategies when the car is moving versus when it's stationary.
"Besides the reverse light switch, I didn't find any other electrical components on the transmission - it's as if there's no longer a connection for the speedometer, and the speed signal probably comes from the ABS system."

So, I pulled the fuse for the ABS control unit's ignition voltage (terminal 15). The instrument cluster responded with a whopping 4 warning lights and an audible alarm, and, as suspected, the speedometer no longer worked while driving.

My dog, however, remained completely unfazed.

After that, I tried listening with the window open: if there's a loud noise inside when the window is closed, nothing comparable comes in from the outside, or the outside noise is drowned out by the engine.

Considering that the 9N, despite what is arguably quite good sound insulation for engine noise according to the IMO (International Maritime Organization), can be extremely sensitive to certain body noises (e.g., swirling sand and gravel almost sound like they're hitting a metal bucket), I'm starting to suspect that it also transmits certain frequency ranges of normal gearbox noises almost undamped into the interior.
Because, especially with the 9N model, the rattling of the gear selector is already a known issue – so far, it's been more common in the older versions with 5-speed transmissions, but why should the 6-speed gearbox, with even more moving parts, be quiet?

(Perhaps there's a silver lining: if the engine noise suddenly sounds different than usual, it might be a sign to avoid areas with poor network coverage or to start looking into replacing the gearbox.)

Even if I come across a suitable tool, I will still check the oil level.

What exactly is meant by "losräder"?
Do certain gears have specific names, or does the term refer to gears in general that are not currently engaged?

}Edit: }
It's resolved; I found something about it on Google: gears that can rotate around their gear shaft.

I also plan to check it - but where can you get such a tool ?
Have you had similar experiences with noises that were caused by insufficient oil?

Also nen großer Innensechskant ist das meist, Dein Getriebe kenne ich explizit aber nicht. Was die VW-Ings da wohl wieder verbrochen haben? Is it possible to take a photo?

As a makeshift solution, you can try inserting two large, flat-head screwdrivers diagonally into the Allen key socket and then turning them. However, you'll need space and patience for this method.

I personally only experience gearbox noises when the bearings of the shafts are worn out, or when production debris remains inside the gearbox. If there is too little oil in the housing, the shifting performance is usually affected, but noises are rare.

However, I suspect there's nothing wrong with the transmission itself, as it seems to be related to the vehicle being stationary. When you disengage the clutch, the gears inside the transmission stop rotating for a longer period.

Attempt: Jack up one wheel, rotate it, and listen carefully. I've experienced this before with Mercedes-Benz vehicles where either a brake line (!) or the brake pad contact point was loose and rubbing against the inside of the rim. This can happen during tire mounting, but it shouldn't cause a rumbling noise.
Possibly, you have encountered a faulty wheel bearing...

Julian wrote:

So, usually it's a large internal hexagon, but I don't know your specific transmission. What did the VW engineers probably do this time? Is it possible to take a photo?

Hi Julian,

I still have a 17mm Allen key left over from my Golf, but it definitely won't fit.
To take a picture, I would need to drive the car onto a ramp and remove the engine block, which I can't do until next week at the earliest.
I didn't want to believe it at first... imagine the serrations of a ZK screw, but with a pin in the middle, like the screws used in the Bosch mass airflow sensor (although the pin on the oil screw would, of course, be much thicker).

Quote:

As a makeshift method, you can also use two large, flat screwdrivers crossed over each other to engage in the inner hexagon, and then turn them. However, you need space and patience.

That probably wouldn't achieve anything, mainly due to the suspected lack of momentum...
Then I would probably try to ream out a standard multi-cusp implant (what type of drill bit would be suitable for that?).

Quote:

If there is too little oil in the housing, the shifting ability is usually the only thing affected, but noises are rare.

It's incredibly easy to use from the very first day *amazing*.

Quote:

Attempt: Jack up a single wheel, rotate it, and listen carefully. With Mercedes-Benz vehicles, I've often encountered situations where either a brake line (!) or the brake pad contact point was loose and rubbing against the inside of the rim. This can happen during tire mounting, but it shouldn't cause a rumbling noise. Possibly, you've encountered a worn-out wheel bearing...

Hmm, I've already removed the wheels several times, but I didn't see anything that could be causing the rubbing (but I need to take a closer look).
I also find the wheel bearing explanation unlikely, because even in the most vulnerable speed range (up to about 10 km/h), it immediately becomes quiet when I turn off the engine.
Wheel bearing noises are unlikely to be impressed by that... ?

Okay, I found the tool for the oil drain plug thanks to a tip from the Ibiza forum (see attachment).
According to the Hazet catalog, this is also used for the Golf 4, so it might be of interest to other people as well.


Currently, the core question related to my project is:

What could cause an irregular rumbling/clattering sound in a running engine, with a rolling speed between approximately 1 and 10 km/h, without power transmission to the gearbox, that is primarily perceived as body vibration?

Maybe someone else has an idea - I'm a bit stuck at the moment.

But by foregoing the usual climate control and other common features, I still have enough money saved from buying the Polo to afford all the necessary splined shafts in gold, if necessary.)

I would rather get a grinding machine with everything included (e.g., a clamping table, a 3-jaw chuck) and hardened steel bolts from a scrap yard. Then you are more flexible, and the people at VAG can come up with new splines until they turn black.

Here's something else. However, it doesn't quite fit your description 100%.
Okay, I'm ready. Please provide the German text you want me to translate.

joegolf wrote:

There was something else. However, it doesn't fit 100% with your description: /viewtopic.php?t=1802

Yep, the main difference:

Quote:

disappears when the clutch pedal is depressed

-> In my case, pressing the clutch pedal doesn't really affect the noise.

I just saw it, but what about the influence of the AGR?
It also causes a change in sound. My ASZ engine in my Golf has exactly the effect you described, practically from the beginning, and I've always attributed it to the EGR system so far.

Sorry, I'm only here rarely these days, and that's why I haven't followed your "AGR-is-suddenly-defective" strategy any further .