For me it's the Pringles shaped plate. It's concave, so the middle of the plate whacks the tailgate. Ordered a rubber frame to deal with it. We'll see how it works. I'll share details if it isn't a waste of time."NJ pringles shaped license plates" LOL
Often times its the license plate which can be fixed with bolts. Once you identify the source it shouldn't be too hard to fix things. Can you tell where its coming from?