2000 Mustang GT Wants to Choke Out

This happens periodically -- more when it's cold, but not exclusively.  Car runs fine then gives off a binding or "moaning" sound as RPM's drop, sometimes to a stall.  Starts back up just fine.  Revving the engine when I first hear this sound clears it up, at least temporarily.  Never happens on the highway in 5th.  Happens in every other gear; clutch in/out doesn't matter.  Happens when idling in neutral or clutch disengaged.
Has anybody run into something like this before?  I used to think it was a bearing going bad that was putting load on the engine, particularly because of the sound.  Now I think it is more an aspiration thing.  I replaced the air filter -- no change.