Hey mate,
I've read through your thread and it sounds like we have the exact same issue. Mine is definitely coming from outside the car so its not the seat or anything like that. This weekend, if I can, I'm going to try and get the car up and see how freely the wheels spin (easier said than done on London streets).
I'm thinking its wheel bearings to be honest, usually this is a kind of groaning sound - but from what I've read on this forum, on M5s this seems to start as the clicking noises we are experiencing. Be good to walk into a garage knowing whats up so they can just get on with fixing it. Keep me posted!
Cheers
Chris