SkyHiCO, drguyacheson, Vaughn, 2G, thank you all for your input.

SkyHiCO, drguyacheson, I'm relieved to learn that I'm not alone. I wonder if there are more pilots having noise with AR4201 radios? Please post to this thread if you are getting noise on 123.5 MHz.

Vaughn, thanks for giving the problem a name. One of the Google hits I found says a birdie usually sounds "like unmodulated carriers -- signals with "dead air." Occasionally they are modulated by clicks, humming sounds, or audible tones." The humming sound fits what I hear.

Tom, your idea of disconnecting the antenna is nice. I know that the 123.5 MHz noise I hear is not coming through the power leads, since I hear it on a radio powered by a battery that feeds no other instruments.

-John, Q3