Looking at a 2004 Grand Prix GT1 with 56,000 miles that has a faint popping feel (like a tiny bump) when put into reverse and then again when put back into drive. Was told that it needs an engine/transmission mount. Plus, the speedometer spins wildly at a moment's notice. (Mostly after a right turn.)
Are the two things related? Does either suggest that the transmission has been removed or otherwise had work done on it?
Just an update, really: The speedometer was set to kilometers rather than mph. Never was a problem. Hadn't realized that it could be changed, but I figured it out. Had it put on a computer and driven by mechanic just in case. It's perfectly fine.
Had the " bump" fixed. There was a loose rod that would hit the bolt whenever the car was put into gear. Bolt was tightened, all better.
Thanks to all who read my question and maybe tried to come up with an idea or two.