Yep, the turbo bypass valve releases excess boost when the throttle is closed. I was noticing a slight high pitch air release sound coming from the turbo area when I turned off my engine and heard it activate 5 min. later. Inside that bypass valve is a rubber valve. When the engine is off and pressure is released. The excess boost is slowly purging from this valve diaphragm. If you notice this sound increasing or you notice less boost from your turbo. Your bypass valve may need to be replaced due to wear.Sounds like the turbos blowing off excess boost.