Noises over 5,000 RPM

When i am right around 5,000 RPM i am hearing noises from under the car. It sounds like it is between the wheel and drivers side door, but i am not possitive. The noises sound like a rattleing. almost like a heat shield rattle.

the problem is that whenever i get under the car, i dont know what to look for, and have not yet found anything lose? Has anyone else experienced this? Any ideas?

busted cats rattling inside the exhaust??

or like you said heat shield. just look for clearance with anything and the frame, engine or pipes.