Hissing sound from driver side dash area
 

I recently replaced the heater core on my 2000 Blazer... all is now good except I'm hearing a quiet hissing sound from the driver side of the dash somewhere. Only happens when the a/c is on. Doesn't seem to be affecting anything other than the noise, from what I can tell.

Not looking forward to taking the dash off again. Can anybody give thoughts as to what it could be? And if I can just leave it awhile? lol

my Blazer makes a similar hissing noise which is a vacuum leak on the vacuum actuated door that opens when i select defrost. So i leave my heater set to dash and no problem. Near as i can tell there is no replacing that diaphragm without pulling the dash.