Squeaks and Rattles underhood and in dash?

[H3 Hummer]

Not sure if this is your problem but you can check or it is good information for all.

Next to the firewall are some long steel tubes that are held in place by plastic connectors. The tube just pops into them. Well if it pops out it will rattle and sound horrible when going over bumps. I took my truck in and they didn't find anything but I found this and now no more issues.