I've had this problem with my car since I bought it and it is doing my head in. The car shows no codes on vagcom and has had new DMF and clutch, injector loom, egr and manifold cleaned and a new MAF, and a new coolant temp sensor. Here's what it does.
In a morning, (weather mild like it is now) it starts first time and drives great, responsive etc. I drive 30 miles to work and when I get there and stop outside the car wobbles at idle, but this will stop if I put all the lights on (fogs, front fogs, the lot).
I work an 8 hour shift and on the way home there is a vibrating noise between 2000rpm and 2100rpm. Doesn't do it in 1st-3rd gear but gets noisy in fourth or fifth, and the car is no where near as responsive as it is in the morning. It doesn't do it if i accelerate hard past 2000rpm through the gears. Its is very noticeable at bang on 70mph in fifth. Now when I get off the motorway approx 8 miles from home I always have to stop at the end of the slip road. When I set off again the noise has all but gone and there's an urgency in the car that wasnt there when I left work. I can also feel a vibrating through the accelerator pedal most days, but again, not in the morning, only after the car has got warm then cooled down. The noise is hard to locate as it only does it on the move under load, but it seems to start at the front of the car and carry to the back? If i go to 80 (on a private road of course) the noise is almost non existent but if i lift off, the noise get worse as the revs approach the 2100 rpm mark, then gets loud at 2000rpm, then stops when the needle dips under 2000rpm. the car has only done 77k miles (had done 52k when i bought it and it had this problem).
On sub zero mornings, the car turns and fires first time but struggles to hold revs for about 10 seconds. I also have a bit of off white smoke, but not when the weather is warm (above zero). I only mention this in case it is related.
I have no smoke, no whistling noises, no loss of MPG, no fault codes of any kind and injectors are within tolerance. I have had the engine and gear mounts checked and they are OK. The engine does rock quite a bit when warm but again, is rock steady if i turn all the lights on. Its also had copious amounts of injector cleaner fired through it. The wobble remains with the clutch depressed so dont think its DMF even though its a new one.
Any ideas appreciated. This is the last shot at getting it right before it goes.