Thanks Barry. I was thinking the same thing about the rattling but the tank is remarkably solid. I can force it to rock back and forth by hand but, otherwise, it sits pretty securely. I'll look for those indents that Justin mentioned and see if I can get some foam pieces in there, temporarily if nothing else. I'll need to get under there again soon so I'm going to check closely for any wear underneath but I don't recall seeing any the last time I had it off.