Vive Tracker Making Whistling Noise After Being Plugged into Computer USB to Charge

[Starfrost]

I was usually able to charge my vive trackers with no issues at all, but now when I plug them in, 2 of them start making a whistling noise. It doesn't seem to affect their ability to charge, but it is a bit strange, annoying and worrying. Does anyone have any clue as to why this happens and whether or not it's bad?