It sounds a lot like it may be a bad wheel bearing. Especially since it has progessively gtten worse. See if you can jack up the rear of the car and spin the wheel in question to reproduce the sound to narrow down possibilities.
Another possible culprit could be a sticking brake caliper or perhaps even something (ie. rock, pebbel, etc) geting stuck between the rotor and pads.
Albo's Official Upgrade Thread: http://www.bimmerfest.com/forums/sho...d.php?t=532572
2003 530i (Alpine White/Sand Beige) - 18x8.5 & 18x9.5 ACS Type II Wheels - BC Racing Coilovers - Eibach Sway Bars - Beastpower Sway Bar Brackets - M-Technic Replica Bumpers - Slimmbones CSL-Style Spltters - Magnaflow Muffler - Dinan CAI - TopSpeed Performance Strut Brace - Carbon Fiber Valve Cover Fascias - Zionsville Autosport Radiator/Fan Kit - Revmax Stage II Torque Converter - AMG Paddle Shifters