BozemanMTS4

I’m stumped as to where I’m losing boost. Pressure testing hasn’t revealed an external leak, and we think it’s venting back into the intake. It sounds like a busted diaphragm somewhere.
Video of noise:

The car lost boost on a short run between on ramps. It ran well down the onramp, but when I pulled away at the next off ramp light, it had the wining noise which occurs once the car builds to 0 psi, and doesn’t build pressure past that. Car is stage 3 with frankenturbos. It peaks at 27 psi and holds to 25 psi normally. It’s not a new build, and has ran well for several years since going stage 3. There is no oil or smoke in the exhaust, and the sudden loss of boost indicates that it’s not a turbo failure, at least in terms of it being the dentist drill sound.

Things we believe we’ve eliminated by swapping parts or inspection:
Diverter Valves
Boost Gauge piping
TBB (Throttle Body Boot)
Solenoid valves N249 (#2 on diagram) and N75 (#51 on diagram) http://www.audidatabase.net/Files/AP...trans_01.5.pdf

Here’s a charge path diagram for reference as well if you know of something specific I should check.
http://www.audidatabase.net/Files/APB_charge_path.pdf

Any help would be appreciated, just trying to get the car back on the road before winter comes.
TIA