Check the front strut mounts. See if the foam "bushing" has deteriorated. If so, it's likely your strut mount washer is rubbing on the chassis. This was the
exact sound we got from our 35d (but applicable to any e70). I have some pictures of what to look for. There are a few threads on the problem here.
Here you go:
https://xoutpost.com/bmw-sav-forums/...ssemblies.html
The "dirt" that you see in the pictures is actually deteriorated foam. The offending washer is that large one under the aluminum brace in the picture. BTW, this will continue to get worse and eventually start knocking. Possibly damage something if in continued use. Take a look. I'm pretty sure that's your issue, but if you're lucky I'm wrong. Probably not a cheap repair if you have a dealer or indy do it.
Edit: That washer is supposed to sit in the middle of the chassis hole for the strut mount. Here's a shot of a failed one vs a replaced one. This is exactly what ours looked like except the washer was actually rubbing against the chassis area. Photos courtesy of Funfdreisig.
