Strange sound when cruising out of gear

Hello guys!

I have Mercedes w211 from 2004, 110 kw e220 cdi and it’s running quite well. Lately when I cruise, starting from 40, 50 km/h and up, and I shift out of gear (it’s manual), I can hear strange sound in the rear.

Once in gear again, nothing, everything perfect.

I can also hear the sound on the top of the acceleration pedal, but just for a second until I press it harder.

Thank you very much for your advices!