[Panzer909]

Try putting the car in neutral with the emergency brake on. Get out, stand outside of the car and walk around it. Does the sound actually seem as if your muffler is bad? Well, I had this same mysterious rattle on my '72 TR6 some time ago. The shift action was fine. I pulled the tranny, and the clutch was in good shape. The throwout bearing, however, was utterly destroyed.

I've heard conflicting messages: some say the grinding when in neutral and under acceleration is fine, but I just cannot see how this can be, as SOMETHING in there is definitely wearing away.