I have the device in use - but don't use audio output via monitor (sounds a bit too thin IMO and I have better alternatives anyway) - but I would never have noticed any whistling sound.

What does Galaxus/Digitec say?

I haven't noticed anything like this so far. What bothers me is the cumbersome and confusing remote control. I wouldn't have bought it if I'd seen it beforehand. I wanted a good display and not a "television".

I only used it for 2 days because I was unhappy with the display, but it didn't make a whistling sound.