I have a rattle between 60-70 mph, no rpm needed (it will rattle in neutral down hill).
The sound is centered under drivers side floorboard right under where the brake pedal would be if you pushed the brake pedal all the way down to the floorboard.
Funny thing is it sounds like it's coming from under the dash, but if I stick my head down there while girlfriend is driving, it's definitely coming from outside the firewall under the brake pedal.
I've banged the catalytic converters, the cat cover shields are not loose and nothing sounds loose inside.. but looking on this forum posts, the catalytic converter is highly suspect.

Any other ideas?

It could be headers or something connecting them.
It's not wheels or tires, and not driveshaft, not torque related since no power needs to be applied for the rattle.

It's literally the trucks speed introducing a harmonic vibration to something else.

1997 F-150 4x4 4.6 liter

 

look into the drivers front tire wheel well for the metal tube that is in the pic and wire tie it to the hose and see if that stops it. i had a noise that you would swear was coming from the dash, floorboard, windshield, everywhere but it was that metal tube rubbing up and down. also look under the edge of the frame right behind the wheel well, there are some of those plastic push pins holding some of the wheel well cover on that might be loose letting it rub a little also.