Okay it just started happening on the way home. When I'm accelerating it's fine but let off the gas and I hear this nasty rattle/zzz sound coming from my exhaust, I think my second cat. Does anyone know what this could be? I know the gasket from the manifold down to the rest of the exhaust blew out the other day. Is it possible that a chunk could have lodged in my exhuast somewhere or something? May sound like a stupid question but a question nonetheless.

 

exhaust leaks are most prominent at decel. could also be a rattle from your heat shields. Get under there and bang on your converter ( when its cold ) if it sounds like there are pieces loose in there, its toast.