Rear End Suspension

97 GP GT 118,000. I'm getting some slapping/knocking noises coming from the rear area when I hit certain bumps or dips in the road. The dips or bumps don't have to be severe to hear the noise and it does not always make the "Pop/knock" noise. I replaced all four struts with KYB's about 10 K's ago and used the existing stock springs (and reused) all four strut mounts, (they looked good). Before I replaced the struts I backed into a cement barrier and slightly bent my rear drivers side exhaust tip. I took the car in to a local muffler shop and they saved my exhaust system by heating it up and bending it back to "around" normal. The drivers side muffler sits a little lower than the passenger side muffler. Today I measured and noted that the rear drivers side is sitting lower than the rest, about a 1/4 of an inch. My question is do springs get "tired" if you will, and would it have been a good idea to change out the springs when I did the struts? Or could this just be my exaust not being in the right position and slapping against the underbody, or a bad mount. etc. etc. I've been under there and cannot isolate where the sound is coming from.
Sorry for the long post but I just wanted to throw this out to you all before I take it in.
Thanks a million...

I see you still have some issues Paul. The 1/4" height difference is within specs. Before you take it in you could verify the three strut plate to body nuts are tight and torqued properly. The noise could be the upper strut mount/load bearing, rear stabilizer links, trailing arms, etc. The strut coil spring can be measured to check for wear and proper height but that must be done off the vehicle with strut assembly fully disassembled.

Rear Suspension Strut Shaft Nut
75 N?m 55 lb ft

Strut Mount to Body Nuts
45 N?m 33 lb ft

For exhaust best thing to do is a visual inspection. Make sure there is sufficient clearance between the exhaust pipes/mufflers and body undercarriage. Also, inspect the rubber hangers and mounts.