TDI Golf 3: Engine Noise at Idle

Hello,

I've recently started experiencing a loud rumbling noise when the engine is idling.
The vehicle has a 90 horsepower TDI engine with the engine code ALE.
The knocking noise only occurs when the engine is idling; as soon as you give it a little gas (approximately 1000 RPM), it disappears immediately. It mainly happens when the engine is cold. When the engine is warm, the noise can be reproduced at idle by turning the steering wheel. This is probably because the engine speed drops slightly due to the power steering pump.
"When the rumbling occurs, and you tilt the engine backward (into the motor mount described here), the rumbling only decreases slightly. It also doesn't necessarily originate from the exhaust system (vibration test). The most noticeable reaction was when shaking the aluminum pipe (located at the rear of the engine) leading to the turbocharger. Overall, the noise seems to be coming more from the front. When you are under the car (in a pit), you can actually hear very little of it."

Who knows the answer???

Please search for "Flexrohr".

Tip, have someone help push the cat upwards while you listen from inside the car.

The noise will probably disappear then.
But also your savings, if you decide to change it.

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

Hello.

I had the same engine and experienced a similar problem. However, in my case, it was a heat shield that was slightly cracked. But based on your description, you've already listened inside the engine compartment, so that possibility is probably ruled out.

Maybe it's just the sound of the belt!

If your alternator has no automatic tensioner and the spring is worn out, the alternator ribbed belt will vibrate a lot. Many of us have experienced this kind of droning or rumbling noise.
The search function provides many entries related to this issue.

Best regards, Georg.

That's right.
A good indicator of a humming sound related to the clutch or freewheel mechanism is its dependence on the number of revolutions.
the connected devices. In my case, I could hear whether the turn signal was currently on or off.
was.

Best regards,
Rudi

@ Rudi and Georg - TDI
Great tip! I'd like to look into that, as I recently replaced the timing belt and tensioner pulley, the ribbed belt, and the V-belt for the power steering. The V-belt for the power steering was too loose after about 5,000 km, so I tightened it.
But a silly question: What do you mean by "free running" on the timing chain? It automatically tensions itself. How can I tell if the tensioner is worn out??? The Golf now has 248,000 km (approximately 154,000 miles) on it.

@euwing
Which heat shield are you referring to (location)? I'd also like to investigate that comment/note.

@dieselmartin
I also suspect the flexible pipe, but unfortunately I haven't found anyone to help me yet .

@ "all"
Today I listened carefully again, even with the engine warmed up: It's loud when the engine is cold, but only audible for a short time after starting the engine when it's warm. It's barely audible under the hood (when the engine is cold), but in the interior, there's a loud rumbling sound (when the engine is cold).
When the engine is cold and there's a booming sound inside the car, giving it a little bit of throttle (around 1000 RPM) makes the booming disappear. It's not noticeable during normal driving!

"Awesome," or what? }

Okay, then go ahead and get under the car by yourself and push the catalytic converter up.
Then you'll also hear a difference when the flexible pipe is damaged, but it won't be as much of a roaring sound because the resonance chamber is essentially infinitely large.

Heat shield plate:
It usually rattles from somewhere near the large hydraulic reservoir in front of the passenger's feet.

Spindle arm:
The tensioner arm of the serpentine belt should have a maximum free play of 1-2 cm when the engine is idling.
Otherwise, replace the tensioner element (it's the black box next to the "back" of the alternator - I have a detailed guide, but unfortunately it's only available offline - I can send it as an attachment if needed).

Free play:
Please search for "Freilauf".
It's like a freewheel on a bicycle; if a bicycle didn't have one, it would make your legs spin uncontrollably if you pedaled quickly, and you wouldn't be able to do anything else.
That's exactly what happens with the LIM (Limb) on the 1Z engine:
It's driven when a cylinder fires. Then it rolls forward, and the motor slows down. However, because the alternator has momentum, it PULLS it along, but on the "slack side" – that is, where the tensioner is located. It then simply pulls it towards the shorter section of the belt, and the tensioner gives way. When the next firing occurs, the motor pulls again on the "correct side," and the tension arm returns to its original position.
Therefore: it flutters.

Does the Lima have a freewheel? If so, can it roll faster than the engine when the engine is in the compression stroke and slowing down?

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.