Surface Pro 4, annoying noise from the back!

I've had my Surface Pro 4 for a couple of weeks and recently started noticing a screeching sound from the back of the device, more obvious from the top of the vents. It's a bit like a very high pitch modem noise. It's not hugely loud but quite noticeable in a quiet environment.

It's not due to wireless or Bluetooth as it continues when in flight mode. Neither is it affected by muting/unmuting the speakers. It's not affected by screen brightness either. It doesn't do it all the time but seems to now be more often than not and I can't be sure whether it's always been like it. When I drag the pen on the screen the noise is partially changed to a white noise sound but the modem-like noise is still there.

Has anyone else noticed this, or could my device be faulty? It's starting to be quite irritating. My wife's Surface 3 also makes a white noise sound when a pen is in dragged on the the screen but doesn't make a screeching noise.

Presumably it's PWM (Pulse Width Modulation). The screen backlight is rapidly switched on and off rather than true dimming. The electronics tend to make a screaming noise as a result unfortunately. I've noticed the keyboard backlighting is even louder. 😩