Yep, something simple and speed-related to the right rear wheel. Has to be brake related IMO. If you don't feel a pulsation/vibration when braking hard, we can eliminate the rotor. Since it's intermittent, and the M3 does not have a separate caliper for the parking brake, you can quickly eliminate the parking brake if it doesn't do it after using it. That leaves a sticky caliper piston IMO.
Do you drive a lot in rain? That'd explain it. With crappy single-piston calipers, if the caliper pins are not lubed properly they tend to stick, and wear the inner pads quicker too.
Don't sweat it buddy; it's something stupid, so don't waste more of your time and just wait until it's consistent then take it in. Or if you're the DIY kind, I'd just remove the caliper, inspect it, clean it, and lube it (along with the parking brake mechanism). Or simply lift the car if it does it when arriving home, turn the wheel by hand and you'd know right away. It could also be other things, like the rotor shield, but it wouldn't have those symptoms IMO, but you can check everything if you decide to remove the wheel yourself (which I'd do for sure
). Good luck, and keep us posted.