07 Monte SS original owner 107,000 miles.
Last spring a small thump,clunk,tap noise developed in the drivers rear. With 90+ miles I decided to do rear struts. Replaced the strut and mounts with OEM.
Noise still present, Then I replaced bushings for rear sway bar along with links. Then Both rear trailing arms, both sets of (control arms?) going side to side. and finally both wheel bearings. Yes while I was under there new rotors and brakes. THERE is nothing left to change. Still the noise.
I dropped the back seat while the wife drove the car around and I could feel the bumping on the strut top.
I don't see any rust on the metal. Tried retightening the strut top nut. I found a TBS to cut the rubber tabs from the coil spring. NO LUCK.
I wonder if the new strut could be bad? I recall it only took like 8-10lbs to push down or extract it.
Tomorrow I plan to tear it all apart hope to find something.

 

Holy hell I have this problem with my bonneville!! Have replaced everything like you said and eventually gave up trying to figure it out.. Let me know if you find anything out pls..

 

Placed cut up rubber spacers between studs and rotor holes. New rotor didn't have the small hole to replace the little screw to hold it in place. This caused slight movement and possible noise between the rotor and studs when driving. Noise is gone!