Rattiling from the break and gas pedals?

[3Hummer]

Anyone have a similar problem? Today on my way back from school my gas and break pedals were rattling and shaking like crazy, and i was able to hear it a little as well. I took it into the dealership and the second I got there the rattiling stopped, so at Hummer they called OnStar and had them run a scan of the car and they said they found nothing wrong with it. After I left the rattiling starting again but only very minimal, If it gets worse I'm going to call OnStar up again and go back to the dealership. But in any case anyone have this happen to them, and does anyone know what it is? Thanks.