Hello and welcome . Is the sound there most (or all ) of the time when the car is moving ?
Very common on C class of this age is blue plastic clip(s) get brittle and let the cable drop onto the drive shaft. You are probably not going to believe me but such a co incidence.
I was under my 2006 C class yesterday fixing this very fault !. It was a complete PITA to access mainly due to the massive exhaust and the close proximity to the secondary cats on the V8 version ,removing the heatshields (another pita) exposes the cable .
The first image shows the cable shroud out of it's clamp , the second shows the broken blue clip and the cable laying on the driveshaft. For reference the large curved metal part in photo 1 id the drive shaft centre bearing , gives you an idea of where to look . Let us know what you find
Yes this is a very very common fault. (assuming its yours)The handbrake cable brakes off the clips holding it up. The result is it gets tapped by the propshaft turning when the car is movng and no the prop shaft is not turning when the car is at a standstill so nothing at idle. It is most likely that. If you think of the old bicycle trick with the peg slapping against the spokes on the wheel to make it sound like a motorbike as you cycle faster or slower. Maybe a crazy way to describe it. Similar situation and result.
Chrissy , remove (or have your mechanic remove) the heat guards underneath the car and compare what you see with the photos in my post #2 .
Let us know what you find.
Note the sound coming from my situation often did not happen while driving in reverse , that I can't explain . But as stated the driveshaft only turns when the car is moving.
Turns out fluid was only ever changed in the gearbox, the bottom of the gearbox hadn't been removed in a long time. So got filter changed and cleaned out sediment sludge. Added new fluid as not enough was ever added back in.