For several months I have been experiencing a loud "ker-plunk" (metal-on-metal?) sound coming from the rear end of my T5-R, particularly at low speeds and over bumps.

This weekend I changed the shock mounts, but the noise remains.

I have IPD springs and Bilsteins all the way around. A dealer did the installation 2 years/15K miles ago. After reaplacing the rear upper shock mount, I was able to tighten the piston nut as tight as it would go-- that is, it felt like all the way to the last thread, but it still didn't seem tight enough. The piston moves from side to side when the rear of the car is rocked up and down. The clunking appears to be coming from this area.

I bolted the piston through the mount with a single large washer and a nut. The piston is smaller than the hole in the mount. Is there any other hardware that goes here to keep the piston centered and prevent it from moving around? Any hardware on the bottom side of the mount? The only thing I saw was a black case and the piston through the hole.

Maybe the shock is bad. Could it be making a loud metallic sound if it failed?

Could the spring have collapsed/compressed such that the height is less than the minimum clearance requirement for the shock?