BPV's fail all the time, lose tension in the mechanism, stick, lose full range or reliable actuation... I've had it happen twice on my old roots blown Celica, so I got in the habit of always having a spare on hand.
Whistling noises, lost boost, vac leak or slight boost at idle... anything like that is usually traceable back to a bad bypass not actuating properly.
Glad it turned out to be not too big a deal -- good luck on final numbers
