High pitched noise coming from AH Mk1?

Bought a used Analog Heat MK1 and when I turned it on I noticed a high pitched noise coming from the actual unit without anything plugged into it. You can’t hear it through the headphones.

It’s pretty quiet but definitely audible and changes pitch and volume when moving through the character wheel, most audible when on the round fuzz and high gain setting.

Had a quick go at running my DN through it and doesn’t seem to be audible or effect the sound through the headphones.

Is this normal? Haven’t owned a bit of kit like this before, wondering if it’s something to do with the analog circuitry?