I recently had dual exhaust installed on my 66 Mustang. The day I picked it up I noticed an odd metallic tapping in reverse that changed with the RPMs. I checked around the driveshaft thinking the installers may of get something too close but I haven't found anything. Today I noticed that if I park the car on level ground or with the nose facing down I get the odd tapping. If I park the car with the nose facing up I don't get the tapping. I checked the fluid levels and they're good. I've had the car a year and only since the exhaust was installed 2 weeks ago I noticed this issue.

It's a stock 289 with a 2 year old rebuilt 3 speed automatic. Has anyone else heard of this problem? I'm totally lost which isn't a suprise, I know nothing about transmissions.

 

check the tranny mount, might be bad causing a misadjusted linkage. and check the linkage itself
if its out of adjustment you can hear the park pawl popping and it will back up

 

check and see if one of your hangers are loose causing the pipes to move.