I also have a muted rattle coming from the right rear on my ‘22 M3P. It started maybe 6-8 weeks ago but mine sounds like it’s coming from the strut area, maybe one of the suspension or sway bar links, maybe the strut itself?
Seems to happen with suspension movement, it’s most noticeable at low speeds since that’s when it’s easiest to hear.
Figured I’d wait until it gets a little worse before bringing it in so it’s really easy for them to t-shoot.