rattling noise

I own a 98 tahoe and I can hear a rattling noise coming from under the hood or engine area when I accelerate. Sometimes I don't hear it and sometimes I do hear it. Sometimes when I accelerate hard I can hear it get louder. Does anyone have this same problem or might know what this could be. I would really appreciate it. Thanks.

I have a rattle that sounded like it came from the transmission, but after having it checked out its just the catalytic converter. One of the inner pipes busted loose and is just flapping around. Its no real harm they said, just really annoying. You can hear it rattle even by striking the tail pipe. Its about $630 to fix at GM, but I think I can find one for cheaper from midas or somewhere. Good luck on diagnosing it.