My other guess would be that the coolant in the engine heats up and then the T-stat opens making a whoosh noise as the water pump is pushing. A coolant flush and T-stat replacement would need to be investigated first. How's the the engine temps before and after the whoosh noise?

 

Another Whoosh for your van year if it has AC.

TSB 24-27-93 91-93 Air Conditioner Compressor Noise.

Disconnect the AC compressor electrical connector and drive around to see if the whoosh is gone. Maybe when the AC clutch kicks on the whoosh happens.

 

My guess is that the Cat converter is on the way out. Seems to me he had a partial blockage that cleared itself and probably lodged downstream somewhere in the muffler where it is no longer impeding exhaust flow, at least not enough to notice.

Good-bye whoosh and power restored.