Surface Pro 4, annoying noise from the back!

The varying high pitch noise you are hearing - especially with the m3 because of its total absence of mechanical noise source from the inside - are originating from the CPU/chipset and likely associated with the power state changes.

If you watch closely, the pitch vary according to the activity - scrolling, touching the screen and so on.

Interesting.  When I scroll, I hear some scratchy type noise - I don't have much of a problem with this, though, because it is like white noise / fan noise which doesn't hurt my ears.  However, from the limited scroll testing I just did, the high-pitched whine didn't change much - or at least it did not change as much as it does when I change the brightness from 100% to 75%.  If the brightness is at 0% or 100%, the high-pitched noise is essentially gone.  Anything in between this, and it hurts my ears.

Never done that with my two m3 when adjusting brightness

You could tell the electrical noise was related to the backlight because the pitch of the noise changed when the display brightness went up and down

The backlight noise is a defect and should be returned.

Its different than electrical noise via CPU.

When the processor is idle or performing light tasks, the down-stepping of the CPU v-core creates a slight electrical buzz. You can tell it is the CPU by giving the processor a significant load (running prime95 for example) and hearing how the pitch of the noise changes.There is nothing you can do about this noise. Some units will be better than others because of the manufacturing process, but I suspect none are perfect.

I also suspect the M3 is worse because it has no fan, and it operates on even lower voltages than the i5. I would make sure what you are hearing is cpu noise and not backlight noise though--the backlight noise is a defect and should be returned as I said.

Unfortunately, I've tested multiple SP4 i5 units for this backlight noise, and it has been present on every unit.  I've had two replacement units for my purchased unit, and every unit I've used has had the backlight noise. I'm not sure if this is present on other units.  I can handle the fan noise and clicking noise of the CPU, but this high-pitched whine from the backlight brightness drives me crazy.

My current i5 is rock solid from noise issue either CPU electrical or backlight that not had either the two m3 I returned.

But I will return my i5 due to other issue the proximity sensor of the screen is broken and defective and not behave normally.

The manual setting of brightness is not working at all and act as automatic brightness setting.

After 3 SP4 unit I'm done for good with surface tablet after 2.5 years having work with free of issue SP2/3 and Surface 3.

MS should improve the quality control on SP4.

Now I'm thinking new thinkpad x1 tablet as alternative proposal.

Could also be the LCD backlight driver. These are often run by switching regulators and the sound would spread across the LCD. Try use the device without running suggested brightness. It could be that on suggested brightness the regulator doesn't stabilise completely before the set point in brightness is changed (this would be pretty simple to fix in firmware, just limit the update rate of the brightness)