Lid or whole bag?Lower rail block may be worn or bag not entirely locked into it.Lids can be adjusted slightly,if not snapping something over lip of lwr bag-like a small plastic hose,old battery drain style hose-may tighten up.
Try pulling bags off and spray rubber on the bottom of bag mount with silicone worked for me. It could also be rubber mount on exhaust . The spray works on both. Heat makes rubber hard and it sqeaks and or rattles.Hope this helps.
Disassemble the latch, start by disassembling the cloth "strap". I'd bet one of those little screws has come loose that holds the latch. Very easy to perform in the middle of the floor in your living room! Nothing to it....and reassemble with blue loctite.
Hmm...well it's only a guess and pretty to do without hearing it for myself, but it could also be undeneath the bag where the rubber rests on top of the metal support bracket. Is it possible you are missing one of the rubber mounting pieces and the bag is hitting the metal bracket? Could happen. I took thebags off once while cleaning and I had to snap the rubber piece back in to place.
I had to replace the springs in the latches to cure the rattle. You may want to look at a new bike to see how the springs should work. They cost about a buck.