New Driveshaft

So I was hearing this strange clunking from the rear of my M3 whenever thier was a shift in the cars momentum. Almost like a major component was loose and banging around. It got worse when the Sport mode was activated and the torque was more severe. After having the dealer look at it they think they need to replace my driveshaft. Yikes. That dont sound pretty or cheap. Thank god for the warranty. Anybody have any thoughts or similar experience. Hopefully that will eliminate the mysterious clunking sound.

Most of the clunking is the diff, and it is pretty normal for the design of the diffs on these cars. SOM eclunk more than others and need to have the diff replaced.

First time I have heard of any driveshaft problems.

Well, I am not sure if this was the same clunk I have heard others complain of since it wasn't occuring during gear shifting but instead with changes in the cars momentum. It was pretty loud and passengers began to notice and ask questions. Nevertheless, I was still surprised that the dealer was going to replace the driveshaft since I have not seen posts with complaints about driveshaft problems. Well, it's under warranty and they will replace with a new driveshaft not a rebuild one so I guess I am ok with their trial and error approach.
Thanks!