Tym C

I was driving home last night, when the light turned green I hit the gas and I heard a rattle nosie. So when I got home I popped the hood and I couldnt hear it while the engine was at idle. When i turned the butterfly I wasnt able to hear it either. So I had my wife come out and power brake so I could try and make the nosie again. When she got the RPM up to about 1g i heard the nosie. You cant hear it from the bay but you can from under the truck. My stream light was dead so I wasnt able to see anything due to it being dark at 5 o'clock. I didnt have a chance to check it this morning either. My thoughts were that either the engine or tranny mount is bad and when the engine torques the exhaust in rattling against something.

Thoughts??