Strut mount and sealing ring
 

There is a faint banging around noise is my right rear quarter when driving. Do strut mounts usually go out in pairs or can one last considerably longer? My left side is good. I just check the torque, all are ok. I jacked car up and shook everything around, no noise. I then put a 2x4 under tire and bounced up and down, noise appeared. Left side doesn't do. Would having a bad sealing ring be likely another possible cause. I noticed it was in rough shape when I had struts off a year or so ago. I replaced the left side. I know, dumb move not replacing when I had the chance.

The ring is the rubber elastic that goes around edge of mount.

You only replaced one shock, not the pair? Bet that feels good at the back.

Are you sure it's not your track control arms, they knock around.

No, sorry. I replaced the springs on both sides. Bilstein HD's were not changed. It was the rubber ring that goes around the edge of the strut mount where it creates a cushion between the mount and body. I only replaced left side. Dumb I know. It looked a bit rough.
Can I tighten the strut mount with the strut disconnect from the body but still in the wheel well? I had a tough time since I couldn't use my torque wrench since I had to put an Allen wrench (or something similar) on the strut shaft to keep from turning. So maybe it just needs to be tightened a bit.
Sorry, mechanics is not my first language.