Sounds like the horn housing is loose (some have "displaced metal fasteners" holding it together), and when you install the cover, it warps the housing just enough to contact the diaphram inside. If it is the depressed metal type, you could use a hand punch tool and tap each one against a flat anvil if they are like rivets...in criss cross sequence, but it sounds like your gonna be beating a dead horse. Can't hurt in either case...