I had the same problem on mine the first time I took it apart... very annoying.
It turned out to be a missing rubber mount that goes on that side panel that sit wedged between the tank and the frame.
This part:
The rubber mount is at the end of the part (skinny end - left side of the picture)
I find that I need to make sure the rubber mount is properly wedged in there (both sides) before tightening the bolt that holds the side panel or the tank will still wobble these days...