Hey there, I’m posting to this thread because I’m experiencing an identical issue. Did the original poster end up figuring it out?
When the depth knob is in the complete counter-clockwise position, there’s no issue. As soon as the depth knob is moved even slightly clockwise, there’s a high pitched drone that pulsates in accordance to the tremolo settings, and is present both when the pedal is activated or bypassed.
For what it’s worth, this is a new issue that has arisen whilst trying to fix a different one. I was hearing a clicking noise every time the effect activated in the square wave setting. In attempt to fix this I carefully reflowed all the solder joints in the power/relay section and the micro-controller. That’s when the high-pitched drone started.