Strange Noise from gas tank area??

About 2 days ago i started noticing a strange vibrating (almost grinding) noise from somewhere back down by the gas tank. I posted something earlier about a trunk rattle, but this is MUCH different. It seems to be comming from under the trunk.

When i step on the gas while driving and the exhaust (magnapack catback)gets louder this very odd noise begins. It happens randomly between 1,000rpms and 2200rpms. . . . . ..It's like a rattle/ vibration with a little groaning noise. Never heard it before.

I thought it might be something with the transmission or clutch, but now i'm starting to think it could be something to do with the exhaust. Maybe it's loose??? I'll crawl under it today and tighten everything down. maybe it'll stop. Just wondering if anyone else has had this problem

anyone?????
I tightened down everything on the catback.....
I took off my rear license plate.....rode around with the trunk open......The vibrating noise is STILL there...... It must be comming from the trunk. It's buggin the hell outta me it's so loud!!! Anyone know what it could be???

I had someone rev the car and i sat in the back....still can't tell where its comming from. I'm pretty sure its not the exhaust rattling or bumping anything. Maybe that plastic cover over the gas tank is rattling. Sounds like its comming from under the trunk (not in it). Maybe the inside of my back bumper is rattling somewhere.......... ...guess i'll take it to the ford dealer (still under warranty) and see if they can find it.