Engine Vibration at Idle: Causes & Fixes

Hello, recently, a Golf IV with an ALH engine has been experiencing an unpleasant rumbling or vibration at idle. However, this only occurs when the engine is warm. When it's cold, everything is normal.
If I increase the speed from 900 RPM to 1000 RPM, the symptom completely disappears and does not reappear even at higher speeds.
VAGCOM doesn't show any error codes, and I can't find anything loose or rattling anywhere. Does anyone have any ideas what it could be?
Thank you very much and best regards!

Can you narrow down the area where it originates from?

Okay, it's very difficult to pinpoint the source - but it seems more likely to be related to the belt or alternator/generator area rather than on the other side of the engine. However, I don't know if it's located at the front or rear of the engine. Pressing down on the engine towards a specific motor mount only slightly changes the noise.

Have you thoroughly listened to the running engine? Have you removed the timing belt cover? Perhaps a worn tensioner? A faulty freewheel on the alternator? Is the splash shield under the valve cover loose? (Open the oil filler cap while the engine is running ), and listen inside.

Thanks for the tips, I'll check them out tomorrow.
I just noticed something new while watching it again.
"There's oil leaking from the oil feed line to the turbocharger. It seems to be dripping down the line. If it hasn't been burned, it must have consumed about half a liter of oil in the last 600 km. I don't know where it's coming from. The line was replaced (after it burst) 10,000 km ago. It's made of metal and looks like new. There's nothing visible on the engine itself. Near where I found the oil on the line is the connection for the EGR valve, which I patched with a piece of metal. But even if the metal plate was pressed down, that shouldn't leak so much... However, there's quite a bit of fresh oil on the lower engine cover, visible from the front, towards the back on the left side, in the foam."

Slowly, I'm getting tired of it. Every 1000 km, something new .

Take a look at your idler pulley. That's the ribbed belt pulley on the alternator.

Is your serpentine belt slipping?

Regards,

Manuel.