I had a similar issue on my 09 328 w/ the Sport Pkg (?)and staggered tires. At about 6500 miles I was get a rumble from my front end but no other effects. At 9k I brought it in, an hour and a half drive, eache way. The Service writer called me into the bay and up on a lift showed me my front tires, inside edge, worn nearly smooth and cupped. The cupping of the tires produced the rumble. Never heard of that before. At 13k I replaced all 4. I chk'd all around, Tire Rack was lowest. I went to the local shop and they matched Tire Racks price. 4 =1300.00. (And they chipped a wheel) 1100.00 just for rears on a 6, OUCH. And I'm contemplating leasing a 6 series, WOW