Maybe. Make sure it's not the speakers though - if it's gone when you play audio, it's not a whining coil but the speakers.
Had it now again, this time whining at around 0.5hz and constantainly.
Almost not hearable as i had Straßenbahn and normal urban noise.
AND the whining from my HP elitebook is way more disturbing, when its plugged in (that sound was next to me as well)
So now to what I faced:
Pyra was charging (1.7A port) and turned off.
I booted and had it running for ~3 min opening an browser with wifi on, so no reason to get higher temperature.
Bat level were at 93%
Squishy sound came out of the device.
As you meantioned i tried to "play music".
Quickest idea to generate sound was to set speakers at 100% and try to add more to generate that systemsound "ping".
I was able to hear the "ping" sound and that the speakers had power for additional ~5sec to make a static noise.
During the static noise I could clearly hear a shifted sound of my 0,5hz whining. More deeper. After these 5sec it returned to normal.
As I still had it wired I removed the cable and sound was immidiatly gone.
Was able to replicate this twice.
Next time (10min later, when i plugged power in again) it was a different sound, more constant and now I started a webstream to generate continuues sound and play around with powerlevel.
Outcome, it's not affected by music. If you have it loud or not, it's still there until you pull the wire.
Then I tried the USB 3 port and I got a different noise here. More a fast ticking but different here.
And on both ports sound appear when its charging or not (just what MATE is showing me)
Hope this helps