I had a rattle from the ladder skid plate. Only at certain engine RPM.
Had to put rubber isolators on the round tube ends without bolts, and also rubber washers on the end with bolts.
That solved the rattle.

Pull your skid plate off and see if sound disappears.

 

When I had the trans in my 06 replaced by GM I ended up with a very similar noise that ended up being on of the exhaust clamps that was loose. Considering you have no cats and a modified exhaust this might not be the case but its worth a look.