Engine? issue
 

Hey y'all,
so, I've been experiencing a problem with my car for the last 3 weeks, sometimes on startup, and after random trips, BUT, only when stationary, the engine starts to vibrate on low RPM, between 750-900, as soon as I accelerate it goes away and no noises or vibration happens, wondering what could be the cause for that.
Last changed my oil about 1k km ago, I'm changing it every 10k with filters 'n everything.

What I noticed is, that, if the AC is off, vibration happens more often, but it's rythmic, no random knocks or vibrations, but in a symphony as the engine runs.
With the AC on it rarely happens, since the AC keeps the RPM a bit more higher, guessing it feeds more power in to it, slightly, but still. Any ideas?
One more thing, with the AC off, fuel consumption sits as allways, around 0.7/H, as soon as the vibration kicks in, it goes up to 1.5/H

The car's an A6 C7 2011 3.0TDI Quattro 204bHP