Rear subframe.. Check that and have a mechanic look at it.. It is a common problem for this cars for the rear subframe to crack... My coupe has 111000km now and still no signs of the cracks.. Anyways, check out the link below it's mostly for the M Coupe but should apply some of them for the roadster.
2006 M5 SMG III | Dinan Stage 1 Software
2003 Escalade 8 Seater of Fun - 232k and still running
2000 M Coupe