There's a heat shield on the exhaust right behind the transmission pan.
At certain rpm's it sounds like fingernails on a chalkboard. It's hard to find, the easiest way is to have someone rev the motor while you look for it. Put on a pair of gloves.
Assuming it's a heat shield, just put a (about 3") radiator clamp on it and tighten it down a bit. That one is there to protect the tranny pan from heat, so don't cut it off and toss it in the trash.
If that isn't it, you might find it with the gas pedal or a rubber mallet, but it sounds like a heat shield.