Maybe I can get some ideas. My M6 is making a new noise. I bought the car from a one owner and it is in perfect shape. Car is at 45k miles, maybe 10k under my ownership. I now have a noise that thumps or at least makes a louder sound when I engage the clutch. Yesterday coming down I-4 I had to slow for traffic and took it out of 6th and coasted at about 70mph. I could hear a random thumping like something is loose in the rear end. It seems like if I put it back in gear it doesn't rattle/thump. This morning leaving the subdivision in 1st I heard something rattling or thumping again at about 15mph but stops when I change gears or stop....is this a sound that maybe the clutch is going. I dont have any leaks or slipping that I can feel?
One more thing that might be a sign, I have had to replace the clutch master cylinder right after I bought it and the dealership said previous owner had shifter replaced earlier too. Any ideas would be greatly appreciated.

 

Check for a broken tranny mount and check your drive shaft u-joints for starters.