What the hell is that noise?

[SniperGT]

I got a new muffler, i think it's a stock one, since my car has been lowered once in a while things rattle, when i lifted my car, i noticed the exhaut pipe is close to the chassis, almost touching it...when im rev the engine to about 4000rpm, i get a wierd rattle noise, i heard it when i was driving and thought it was the exhaust pipe, then i reved it while in my driveway and heard it. could it be my transmission? the car tuns fine until i get to 4000prm and get the noise but it doesnt feel like its damaged just makes an ugly noise.