This is on an '02 A6 1.9TDI with about 80K miles.
On saturday I noticed that the engine seemed to be idling rough, and that the vibration could be felt through the car.
I checked under the bonnet and the vibration was such that the writing on the engine cover was almost too burred to read (felt like you had double vision)
On sunday morning I thought I'd have another check and it started nicely and idled as cleanly as it ever had, but once the engine was warm the unpleasant idle was back again.
The engine is not down on power, the fuel consumption seems normal.
From what I've been able to find out vibration at idle is either caused by:
Dual mass flywheel wear.
Freewheeling alternator pulley.
Dirty MAF sensor.
Are there any other potential causes that might be related to the engine's temperature?