thanks for reply, guys. First time I have noticed this was about year ago or so. Didn't pay much attention since everyone say that M54 is noisy from factory. Sound got louder and is more noticeable when car is hot and when I tap accelerator like in the video. Dealer had a look, but they said that engine sounds fine to them. Belts are fine, no coolant or oil loss.
Tomorrow I will go and get tensioner piston for the start. I will write once I change it and if there will be any change.