I've been tackling this progressing issue and I now am more inclined to believe it is coming from the turbo. Since my last post on this issue, I've replaced the converter, cleaned the throttle body and rechecked the turbo hoses.
The Virginian Volvo shop that I purchased the car from also checked out the turbo internals and noticed that most of the hoses had been recently replaced.
Is it possible that the turbo was damaged by overheating? The radiator and water pump were replaced a few thousand miles before the purchase, raising that suspicion.
The turbo has to be well into the boost stage for the rattle to occur when decelerating. If I slow down to near idle afterward, the car will also stumble or stall. Otherwise the motor is running great. As I mentioned in a previous post it sounds like ball bearings rattling in a can.
I'm running out of options other than having the turbo rebuilt. Has anyone run into a similar problem with these Mitsubishi turbos?