Had to chime in here after hearing "but the plural of anecdote is data, right??" So true.

You'd think this would be, as the OP hints, a common symptom with a single solution. It might be -- I've heard of it often, but not experienced it myself exactly as reported.

What I've had years ago is the leak a tennis ball fixes. The recirc flap actuator dry rots and that's so difficult to reach to fix properly, wedging something to keep the flap in place against its spring is more likely a daily driver solution. To stop the hissing, just keep the recirculate button pressed in, which cuts off the flow.

Those check valves in the engine compartment seem to almost fall apart in my experience, and soapy water is my choice of cleaning solution, which has fixed the one way valve in every case I've tried, but I think the symptom of failure is not a hissing inside, but a change in function (airflow) when the throttle gets opened.

Best advice has, in my opinion, already been given. OP should maybe abandon the hope of skipping to the "common symptom with single solution" guesswork and sniff around a bit more to see how the climate system's vacuum switches alter the symptom.


--
Art Benstein near Baltimore

"the worst data are better than the best theory," said Antonio Ereditato