Whistle noise at highway speed - Stops when AC is off recirc

I have a loud whistle noise coming from the Evap area at highway speeds. It goes away when on anything other than recirculation selection on the AC.
What could be causing this?

https://www.youtube.com/watch?v=jGRs1O2_UNY

There's a big flapper door under the passenger side of the dash, activated by a vacuum diaphragm. My guess is the whistle is something with the vacuum hose or the diaphragm.

ehall said:

There's a big flapper door under the passenger side of the dash, activated by a vacuum diaphragm. My guess is the whistle is something with the vacuum hose or the diaphragm.

Click to expand...

It's at speed, so not a vacuum leak.

What diaphragm are you referring to?

The HVAC system has a bunch of vacuum actuators that direct air flow dash, feet, defrost, recirc or outside air. I'd guess the recirc one isn't working or weak and when you're going at speed its allowing outside air in slightly causing a whistling noise. The diaphragms are part of the actuator.