Weird Harmonics? A Whistling HP 6830 Printer?

I have a HP 6830 printer.

It is making a high-pitched, barely audible, but just enough to be annoying (to me anyway) whistling/singing noise, that sounds a bit like modem noise.

The frequency and pitch of the noise changes when it’s wifi client is switched off. It’s still there, just slightly different.

The whistling noise is still present even when the printer is in standby.

It changes pitch again when it is powered down.

It goes completely once the mains lead is removed, and returns when the mains lead is plugged back in.

Sadly, I’ve tried recording it, but nothing I’ve got seems to be able to pick it up clearly.

Anyone else got one and come across this?