Hello, I have 08 E350 4matic, and someday this kind or groaning noice appeared.
After cold start, it doesn’t make sound but after I drove car (when engine warmed up) noise appears.
P/s fluid is light brown, topped up.
Noise disappears when i rev up more than 1000 rpm, I don't know why, maybe engine sound is bigger than small groaning noise?

But yeah, should I get some Lubrizol 6178?
https://youtu.be/LjkxWfuzPWo

you can hear the sound when you turn your volume to max. Only hear able inside, not outside of car.

Any thoughts?