I think one of my Jaguars ate a chipmunk.

[EHaskins]

Originally Posted by EricVanWyk

The source of the PD squeal is the 12V (and sometimes 24V) boost supplies. Nominally, the boost supply runs at ~600kHz - way out of the audible range. What you are hearing is somewhere in the 20kHz region(ish).

The boost circuit (inductor specifically) was sized to be able to handle the maximum stress conditions. Since it has a fixed frequency and a minimum on time, there is a minimum amount of work it can do in any given cycle. If it is asked to do less, it will simply go to sleep for a few cycles before trying again. This napping or "pulse skipping" happens in the audible range.

Obnoxious? Yes.
Bad? No.

Thanks for the details.