I have a Comptech Novi1000 on my AP1. Recently, I've noticed that it's making a whirring or low pitched noise above 3500-4000 RPM that only occurs when the throttle blade is closed. It's most audible from the air filter. It sounds like a large fan blowing. I don't notice any difference in boost or performance.

Could this be related to a bad bypass valve? It seems like it must be something caused by the pressure build-up when the throttle blade has air flow to the engine blocked off.

 

YES. In my case it was the Bosch bypass valve that I got on Amazon. Turned out to be a knockoff unfortunately. Immediately failed and it was exactly as you described.

edit: I no longer purchase, even reputable, parts on Amazon due to knock-offs.

 

Thanks, confirmed that mine was also a failed Bosch bypass valve. Noise went away with a replacement.