I have a 2002 F-150 and as I'm driving I get a noise that sounds like something is hitting underneath the cab.
At first I thought it was gas sloshing in the tank,(that's what it sounds like to me)
so I filled the tank and still get a noise as described.
Any thoughts?

 

Likely is the heat shields on one of the Cat converters has come loose from the weld and making a sheet metal type noise.
Use a large strap clamp around the whole cat to hold it in place, even if you have to put more than one together to get the length needed.
Good luck.

 

Thanks! I'll check it out.

 

Turned out to be the front shocks.
I did notice the heat shield was loose, but the noise was a thud as opposed to a tin-like sound.
Thanks for your help.