Bassoon noise when off throttle and when turning off engine

Bassoon noise when off throttle and when turning off engine - Solved!

Mostly I hear the sound immediately after I turn the engine off. It sounds like a low note on a reed instrument. Sometimes it's just an eighth note, and sometimes it drags on for a whole measure. It doesn't happen every time I drive. It refused to make a sound when I had the hood open trying to track it down... I've heard it with the A/C on and with the A/C off.
I thought it might be a vacuum leak, and so I pulled the hose from the valve cover reservoir to test it. There was still vacuum held in there a few hours after the car was last used.
The EGR is new. The ASV sounds OK and does the normal flip/flip sound when I turn off the engine.
A couple of times when I turned the car on just to move it in or out of the garage, I heard the sound each time I let off the throttle. I might have heard it while I lightly on the throttle, too, but I'm not certain.
Any ideas or suggestions? The car has only 215k miles. Original cam.

I believe that it is the N75 valve orchestra releasing the vacuum to the turbo actuator.
I've heard this on both MK4 and MK5 cars.

Makes sense. It fits also with the recent switch to the GDE tune. The tune probably changed the turbo actuator map.

Solved! I checked the vacuum system more thoroughly, and I found two problems. The check valve just behind the valve cover was bad. The noise was still present after I replaced it - maybe even worse. The next issue was a leaky N75 valve, and I haven’t heard the noise sense replacing it.