Ravey Head

Hi, I drive a 2001 ES300. While driving with the A/C on, I hear a high pitched noise about every 1-1.5 minutes that lasts about 15 - 20 seconds. It's regular and always happens at about that interval for that duration. The noise itself is steady and probably around 8-10khz. I've had passengers verify that they can hear the noise as well. Any clues as to what it could be? Thanks!