Railgun

Ok, here's one that I couldn't find anywhere. I have the typical squeeking that I'll look into fixing, but I have another problem. At highway speeds (particularly 75+) with the windows down, the rear hatch seems to bounce, enough that it seems that the whole piece is lifting up to let air escape from the seal. The latch is bottomed out. Anyone else have this issue and have any ideas how to correct it?

Thanks!