Hi everyone, my first posting here. Found you via a google search as I wanted to see if anyone else had this problem. Low and behold, you have....
I have a completely stock 2011 E93 M3 convertible. Upon shifting quickly into 2nd last week with traction control on, i heard a loud "thud" at the rear of the car. driving at that point seemed fine until I let off or applied throttle. Each application would produce the "thud" so I knew something must have let loose.
Dealership looked at the underneath and service manager immediately pulled out a broken bolt - snapped off! He asked if I had any work done on the car and I have not. Completely stock with 13k miles. He says the probable culprit is the traction control under full power. He feels that the time between the tires wanting to spin and the brakes clamping creates enormous torque that is absorbed by these bolts. Either that or they were improperly torqued from the factory.
I could tell there was slight resistance from him wanting to replace the entire subframe due to the bolt "hollowing out" its mounting point. But considering it's completely stock, he agreed and the replacement was covered under the warranty.
Just wanted to pass this on so you wouldn't feel you were the only one like I did.