catalytic converter

Here's one I haven't seen yet. I have a flutter which turns into a rattle when I put my 2000 trooper into any gear but park. It started as a very (almost inaudible) flutter about 6 months ago and has gotten progressively louder since then. It rattles until about 40 MPH and then disappears. It also lessens on longer rides. No change in the performance of the truck. I was told it could be the catalytic converter. Don't know about that. Any other thoughts?:dunno:

I had a heat shield rust/work its way loose on my Integra. Made kinda the same type noise. Its worth sliding uder the truck and pulling on the exhaust parts BEFORE you drive or AFTER its had plenty of time to cool down. Let us know what it ends up being.
Tom