Exhaust pipes are notorious for banging against the frame, differential, etc. Noises, like water leaks are sometimes decieving. If you have checked underneath and are certain there is nothing that can bang against another item, the sound you hear may not be coming from the area you suspect. Check for things rolling around inside, under the bed, in a compartment, etc. I know it sounds like it is wind related and it may be. Could it be something on your roof? A loose AC cover, antenna?
Good Luck with your search.
Bob K from PA