squeak only audible inside when a/c is on

This has been driving me crazy. When I turn on the a/c, once the compressor has been on for about 1-2 seconds there is this squeal sound that seems to be coming from the passenger side, under the dash. It is constant and lasts for about 3-5 seconds every time the compressor cycles on. It doesn't affect the performance of the a/c, and only happens with the compressor. And no matter how hard I try, I cannot detect the sound at ALL from outside the vehicle, including with my head in the hood. Sometimes it doesn't squeal, but I would say 90% of the time as soon as the clutch engages, it will squeal a few seconds later for a few seconds. I should mention that it is not a screech, nor very loud, but definitely there. Most posts seem to deal with an outside noise, so I'm stumped. Any suggestions?