Thumping noise when changing gears?
When I change gears to D or R, I hear a thumping noise every 2 seconds. It's perfectly fine though when it's idling in park and even when I rev it. The noise only happens when I go to a different gear.
I had my Dad sit inside and have him change to R and D. First thing I noticed was the engine block rumbling rough (shaking every 2 seconds). I also bent down to get a look at the underbelly and I saw the drive train moving slightly back and forth clockwise and counter clockwise, corresponding with the thumping noise. My Dad also said while even in park something felt off (not enough power) - which is probably a totally unrelated problem.
Any ideas on what's going on? I'm probably going to get it towed to my mechanic anyway. It's still good to have an idea of what's going on first.
It's worth noting that it has been sitting for a while (like 2 months) and I didn't hear this noise before that. I also recently finished putting in new mufflers and this is basically one of the first start ups. The very start up after the two months and muffler install 2-3 weeks ago, it was fine in all gears.
1967 Chevrolet Impala 4 door "post" sedan
327 5.4L V8 - 3-speed TH350