Ya could be many things.
I do winter to summer tire swaps, and always check arms and links and so on. Nothing looked out of place over the last couple of years. Additionally, I had taken it in 3 times in the last year for alignment due to strange rear end swaying, swerving, instability...I believe this was the initial signs something was going on. 1st time I was told alignment was out of spec, they "fixed it"...but issue still persisted although it was a bit more muted. 2nd time, same response from Service Center, but told me to get new and better tires and possibly one of my rear wheels were out of round. I went ahead and did just that. Purchased a new set of wheels and brand new tires. 3rd time, took it in they did alignment, and I asked them to specifically check if all bolts were tighten in the rear. They checked and all seemed fine, but they noted I probably will need new rear camber and toe arms soon as those bushings were not in the best shape but within spec right now. On this last visit too, on my way home I noticed a clunking noise coming from the rear passenger, took it back and the same tech took the car for a ride and said he thinks its just the suspension settling after his work. Welp, got home and found that the toe arm alignment bolts were both very loose, like loose enough for my 5 year old to turn easily with the wrench. I lightly tighten it, as to not mess with the adjustment bolt...then took it back and they fix it and the clunking noise was gone.