The *POP* you hear is the mode door (defrost/panel) actuator inside the HVAC box "slapping" closed. You folks with this sound have a different problem than just the vac. check valve. Your problem is inside the cab. It may be vacuum related, but inside and NOT under the hood. The door may be hanging up and then suddenly closing. Or, a weak motor that lets it open under load and then slap shut when off load. Maybe even the arm hanging up at some point and then letting go.
To repair this (if not simply a vac. hose disconnect) the panel has to come down (or off) to get at the actuator. However, the HVAC box does NOT have to come off.
WC

 

Replaced the check valve with the bigger "cannister" type yesterday and my symptoms are gone, the TSB only included up to 1996, but it worked great on my 99 and its what the dealership recommended.