My 99 M750 sounded like a dumptruck when I applied the rear brake at anything less the 50mph, and I just couldn't quiet it. to be honest, I didn't use it that often, but when I did, it was embarassing as hell.
I've found tht replacing the push pin with a bolt that just barely fits is the only way to "cure" the noise. I think it was a 1/4 in. allen head bolt with two nuts on the backside, I haven't heard the noise in almost 4 years now.
I've had much the same experience as Chris. changing pads seems OK for a while, but eventually the noise comes back. I've moved to a non- sintered pad which helped, but will probably wear at a greater rate, plus doesn't quite have the same perceived "grip" as a sintered one. Also, I found my rear disc undersize in thickness, and on removal from the wheel it warped really badly (radial rather than circumferential) and seemed to have been "off-centre", running more towards one caliper piston than the other. The new disc sits nice and central in the caliper, and so far (c. 5000 kms) is nice and quiet.