Conch shell sound after AC Repair

My AC died about three weeks ago and the evaporator core and expansion valve were replaced, the AC system was flushed, etc. All was well for about two days, when a strange conch shell sound started coming from what seems to be the glove box area. Took the car back to mechanic who insisted the sound was from the blower motor which needed to be replaced. Blower motor was replaced and within days the sound started again. The sound is intermittent – sometimes days go by without the sound – and literally sounds like a conch shell. On two occassions the sound continued after I turned the car off, and I have to start the car again so that the sound can run its course (usually just about 4 - 5 seconds). Any clues or theories? Could it have something to do with the AC repair? I fear taking it back to my mechanic who admits he has no idea what’s going on.