rattling sound

Well to start off I have a '02 Tahoe LS, I have had the steering klunk fixed, but on many different occasions I hear a rattling, vibrating sound coming from what sounds like to me under the dashboard. I recall hearing it before I had the steering klunk fixed. I have addressed it everytime I go for an oil change, but they need to hear to know what it is, and it doesn't make the sound when I need it too. It sounds like it has something to do with the steering. I have banged on the two o'clock position on the steering wheel and sometimes can emulate the sound. If I drive on a bumpy road(not pot hole bumpy, just abrupt dips or raises in the road) it will make the sound and continue sometimes without going over bumps. I don't know what it is, I am tempted to keep a tape recoder in the car so I can catch it. Any ideas what it is would certainly help

I have the steering clunk to. Just when you put into drive. Only one clunk per transmission engagement. But regarding the rattle.....have someone drive sit in the passenger seat. Put your ear up to the large center ac vent and listen for the rattle...can you hear it? Call me at 888 871 7084 if you do...kind of a squeak....