If the check engine light came on there will be a code. Even if the light is not on anymore, as long as the batt has not been disconnected, there will be a code, even if its a history code. Autozone may not be able to check your car since you have a 92, which is OBD I. They will only read OBD II systems. Try it yourself with a paperclip. You may have an ECM or wiring problem if you cannot retrieve codes.
The hiss behind the dash is most likely a vacuum problem. Do you loose vent heat when the hiss appears? By default, with no vacuum, you will have defrost no matter where the selector is set. I would not think the vacuum issue would cause the check engine light to come on. I have seen many cases where the vac line was unplugged completely and the light never came on. Its easy to knock off when you change the cap and rotor. If the light is coming on due to vacuum I would think you have a more severe problem somewhere else. But as stated earlier, get the codes first and we can take it from there.