Sub-frame. The first Achilles heel of the E46 M3 is the dreaded sub-frame. In a nutshell the body was bolted to the subframe and over time the body that is bolted to the frame would tear. The only way to remedy this is to re-inforce the sub-frame area with new plates. This unfortunately is a costly fix and cost a few thousands of dollars depending on the shop you go to. Every E46 M3 at some point needs this fix done, and should be done as a preventive matter (not after the fact when you see a tear).