I would guess a defect.
My device works perfectly.

I had another defect : most of the time there was no sound at all, from whichever one, and when sounds came, the volume was not adjustable. After a repair everything works now

I've had mine in operation for just under a year. I don't have the problem you describe. Probably a defect.

Our children also hear a high-pitched whistling noise from the appliance. Could your appliance be repaired?

I have been using the product for over a year. No problems at all

Sounds like overheating. Is there enough distance to the rear? Otherwise defective

Many thanks for the answers. I have now triggered a service order :-).
Greetings