This thread is a great idea, especially for people like me that are new to the E9x M3. May I make a suggestion/request? Would the OP be willing to update the original post and summarize what has been discussed? Something like this:
Exhaust- Exhaust leak sound on cold start - NORMAL
Interior Rattles- Passenger door - NOT NORMAL - see post 26 for more info
Engine bay- Sizzling sound after shut off - NORMAL - caused by the air conditioning condenser dripping
Maybe then this could be made a sticky.