I just bought an FD and its essentially stock except for an Apexi intake. And in between shifts it makes this high pitched sound like a pfffffffft. Another RX7 owner road in my car and they thought there was nothing wrong but it makes a weird sound with the intake. It sounds almost like a weird blow off valve...but there is none on the car.

I did search...but i can't respond because of the 5 post restriction. I'll take a youtube video today and post it up. Maybe I have a bad solenoid? or need my vac lines re-done...or it could be the flapper in the Y pipe cause boost comes on ultra strong when the 2nd turbo kicks in.

If theres anyone local thats rotary mechanically inclined your help would be greatly appreciated. I'm located in the south bay. Torrance/Lomita/Redondo Beach

 

See if your stock BOV is plugged into the intake. It's possible to unplug it and get the BOV noise; perhaps the previous owner had done that.

Dave