Hey Everyone,
Thanks for posting!
Hackaday has an interesting article on this I’d recommend checking out. Some of the older (closer to the original) models of the PoE+ HATs are still around, and occasionally these teething issues manage to make it through the manufacturer QC
The problems that have been identified are: high temperature, high power draw from the HAT itself at idle, the 1.2 amp USB limit, a long bolt that contacts the camera connector, a louder fan, and odd behavior when powering the Pi and HAT over the USB C connector.
As Oliver mentioned earlier, the coil whine from that inductor does get quite loud when it’s also being powered by another source such as USB-C (which I can see isn’t the case here, but thought I’d mention the reasoning for it). The Ubiquiti UniFi 16-Port 150W Switch you mentioned shouldn’t be too noisy of a power source to use, but from my experiences with these HATs that often will have quite a significant impact on the volume and pitches of the noise you get from that coil:
Switchmode power supplies that aren’t enclosed are fairly notorious for this, it seems that the QC on your PoE+ wasn’t quite up to scratch from what I’ve heard, unfortunately.
The Support team at Core Electronics should be able to sort this out for you. If you reply to your order confirmation email we’ll be in touch from there to organise a solution 
(If you already have we’ll be in contact shortly)
