My truck started making a funny noise late last week. It sounded like the serpentine belt squawking so I replaced it. Still squawks. Took the belt off tonite to see if it it might be a bearing in one of the accesories acting up. Still squawking. It is very noticeable in Park ans Neutral, but diminishes some when in gear and foot on the brake (approx 500 to 600 rpm).

Any ideas?

 

Check your exhaust system for loose or maybe broken hangers. Also check if anything is riding anywhere on the exhaust. Then check for broken/loose engine mounts.
Is your heat shield secure?
Let us know when you find the problem and what you did to fix it.