If the "cover" is rattling, the whole airbag assembly is probably rattling loose of its mount.
I'd recommend disconnecting the negative battery terminal and waiting half an hour or so before messing around with the airbag. Safe before sorry and all that.
On the left side of the steering wheel is a small, circular plastic tab. Get a small flathead screwdriver, insert it into the small slot, and pop the tab out. There's a bolt behind the tab. This bolt and one like it on the other side mount the airbag assembly to the horn controls inside the steering wheel.
Get a 3/8" socket (it's around 11mm size, just try sizes till one fits) and see if you can't tighten the bolt up. If it is loose, it'll cause the rattling of your airbag.