I have a 1993 Bonneville non */c and when I have the HVAC selected to the dash vents (not defrost vents or heat vents), a loud groaning/fart sound comes from under the dash under moderate to heavy acceleration. I am somewhat of a gearhead, and am almost certain this is a result of loss of manifold vacuum to the door actuator while the engine is under load. Is there a check valve or something similar with a high fail rate? Thx for the info guys!

The vacuum canister in your year is on the firewall next to the wiper motor. The check valve is usually about 2" or so away from it. Check all the lines to it from the motor. It is common for them to degrade over time.

I looked for the can around the wiper motor, but all there was was the vac booster. My car is 1 1993 SE n/a. Is there any other place it may be?