"Ringing" sound from Pegasus Powerbox when used with Celestron Dew Heater Ring

[microbe]

I have recently purchased a Celestron Dew Heater Ring (9.25") and a Pegasus Powerbox Advance (Gen2). However, when I run the heater ring at anything from about 30% to 90% there is a pronounced "ringing" sound coming from the powerbox. The sound reminded me of that which can be heard from a mains lighting dimmer switch, but at a higher pitch.

There is no sound when using traditional dew straps alone. Of course it returns as soon as the dew ring is also attached.
The sound is at its worst when the ring is attached to a dew heater output of the powerbox.
When instead used with an in-line PWM LED dimmer attached to a 12V output of the powerbox, the noise is less, although still noticeable. Interestingly, the noise is also present if the dimmer/heater ring is connected to the same observatory 12V supply before its connection to the powerbox, although is further reduced.

I also have a Gen1 version of the powerbox, which again produces some noise, but it is significantly less that the Gen2 powerbox.

The fact that there is still noise, albeit it less, even when the heater ring is connected to a non-powerbox 12V supply suggests to me that there could be some type of feed back or interference that is affecting the powerbox, and that this is more pronounced in the Gen2 version.

So, my question is has anyone else experienced this issue? Or indeed, is using a Celestron heater ring with a Pegasus Powerbox without this issue? Or is there an incompatibility between the heater ring and the powerbox? Or, is there a fault with the heater ring?

I have Googled for some answers, but to no avail.

I suppose that I could power the heater ring from a completely different 12V supply, but that rather defeats the point in having a powerbox.

Any thoughts?

Adrian