I have as 2004 Chevy Siverado 4x4, manual trans with the 4.8 V8. Two weeks ago I heard an awful rattling noise almost like a heat shield rattling off. I only heard it when the truck was cold and going slow, once it got warmed up it seemed like it went away. Then I didn't hear it for a week. Today I have heard it almost everywhere I go fast or slow! Any Ideas?

 

Possibly catalytic converter. Muffler shop should be able to inspect it for free.