It seems I've read that Astros are prone to exhaust leakage usually via cracked manifolds. I found it in another forum many years ago.
Well, at idle, it definitely sounds like I have an exhaust leak. It's a very distinctive sound...
I can't find it though!

 

I've had seven of these vans and never had a cracked manifold. Where I have had exhaust leaks is the junction between the headers and the Y-pipe, which is usually an easy fix.