[Elecraft] K3S audible squeal

[ae0mm]

My K3S (100w PA, no subRX, no ATU) has an audible sound (from inside the chassis) that is reminiscent of a flyback transformer squeal. The sound diminishes or completely goes away when using GNUradio’s rigctld + software panadapter. Is it possible the KIO3B stops making noise during high polling rates? I’m not certain what causes the noise, and it may not even be coming from the IO board. I’ve heard this squeal regardless of PSU type. (switching and linear PSUs)

If I change CONFIG:RS232 from USB to 38400 b the squeal becomes louder. Disconnecting the USB cable has no impact.

Any ideas what to do to isolate the audible noise and eliminate the squeal?