Strut mount and sealing ring

[DWBOX2000]

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.