Clattering sound from the engine compartment of a 2.5 TDI AAT. Air conditioning?

I have a problem. I own an Audi A6, 2.5 TDI, C4, manufactured in 1996, engine code: AAT. When the engine is idling and the air conditioning is turned on, something is making a very loud rattling noise, so loud that it seems like something is about to fall out of the engine compartment. Even when I slightly increase the engine speed, the noise disappears. Also, when the air conditioning is turned on and I slightly reduce the tension on the belt, the noise disappears. Furthermore, I have observed that there are very strange vibrations in the spring of the tensioner at the top of the engine block. I have read on many websites that the magnetic clutch of the compressor may be the problem. A new clutch is estimated to cost 250 EUR. My VAG dealer told me: It is likely that the pulley on the crankshaft is the problem. The pulley is supposed to cost 230 EUR. I had a similar problem a few years ago with an Audi 100 TDI, but without air conditioning. In that case, it was the viscous coupling on the fan that failed. The cost for this was 160 EUR. Who can help me further? Because the clattering is so loud that you can't hear where it's coming from.

Hello,

If the air conditioner is turned off, then you can't hear any noises, right?

When the air conditioner is turned off, there is still a faint rattling sound.

Wow, I'm having the exact same 'problem'. Not always, but relatively often: When idling, there is rattling and clattering, which occurs as soon as the air conditioning compressor is turned on.
What could it be? The magnetic coupling seems to be intact; it actually grips well. You can notice this when switching from Eco to Auto mode, and there's a slight jerk when the compressor starts. The cooling performance is also fine. The timing belt chain seems a bit loose, but the mechanic says that's okay, and besides, the tensioner should automatically tighten it correctly. The compressor is, after all, an axial piston pump, so could there be something rattling at idle speed?
By the way, it's been rattling for quite some time now, since the beginning of the year and at 30,000 km, so I've gotten used to it, but I would still like to know where it's coming from.

My little one sometimes makes noises too. However, I haven't been able to pinpoint its exact location yet. I suspect the exhaust system because the noise is more noticeable since I installed a new, lighter catalytic converter, which has shifted the resonance frequency.
Maybe it's also related to the fact that the engine with air conditioning has to work harder, and therefore the torque spikes are a bit more noticeable with each ignition. The stimulating force that would cause something to vibrate would also be higher.

Now it's also rattling without the air conditioner. If I press the light machine, it becomes quieter. I will probably have to deal with the problem more intensively this week.
See you later
Heino

Hi,

Could it be that the issue lies with the spool?
During the AAT, I hadn't encountered this problem before.
But with the 1.8T in the Golf 3, perhaps. Also, especially with active AC, i.e., with load.
After the change (due to another reason), this rattling was also gone.

P.S:
However, I'm reading that you have a relatively relaxed situation with the LiMa, and I don't want to create new problems.

The cause of the rattling noise was the generator's pulley wheel.

It features its own vibration damping system in the form of a metallic outer sleeve, on which the belt runs; a rubber core, which is connected to the outer sleeve and the inner sleeve, which in turn is firmly screwed to the generator shaft. In order for the entire assembly to remain centered, there is also a ball bearing located between the outer and inner sleeves.

In my case, the rubber core was torn, so that the outer and inner sleeves constantly oscillated back and forth between their end stops when idling. The entire noise changed noticeably with the operation of the air conditioning system, both when it was turned on and off.

The diagnosis is straightforward: Turn the engine off; hold the outer shell or belt and try to rotate the generator pulley relative to the fixed pulley. If you can determine a noticeable amount of play by hand, the pulley wheel is damaged. In this case, there is an Audi model without this vibration damper.

Perhaps this will help.
Frank