garbology101

I own an '02 GS300, and I've been dealing with with this rattling sound that occurs seemingly when it wants to. It occurs when I start the vehicle, or when waiting at a red light (sometimes). When I turn on the A/C, same rattle, took it to my mechanic and he removed what I believe to be the cat converter cover and it's still rattles. He told me to bring it back and he would check the exaust. I'm new to the forum and would really appriciate some input or advice, thanks.