Allen -
Have you tried tightening the corner latches? If they are quite loose, it might allow the shells to move more than they should. Sorry, can't think of anything that specifically needs to be lubed in that area.
If it were me, I would probably just take a shotgun approach. Using a can of Teflon dry lube from Lowes (Item #: 213197), lube something, see if it helps, lube something else, etc. Eventually you'll find the source. In the meantime, the Teflon lube won't hurt anything as long as you keep it off any fabric. I like the Teflon better than silicone lubes because I think it is cleaner and the Teflon sticks better. But for a shotgun approach, the silicone might work just as well.
Bill
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