I just purchased my FOURTH Pi from Adafruit. These are the neatest little toys! However, they seem to find their way into "production" applications too fast. I am using one as a gateway/router/firewall/vpn sharing box on my LAN, the second as a dedicated Bit-torrent client, and the third as a jukebox in my workshop. Now I again have one to play with - another 3B+.
I noticed that when I shut down the newest Pi I hear a not very loud but distinct high pitch squeal. Sounds like the mating call of a dial up modem only higher in frequency. Sort of nostalgic I have traced the source of the sound to the power supply. It is the same 2.5A Adafruit supply as I have in use on the other 3 Pi's.
If I plug in the power supply without the Pi there is no sound. Nor is there any sound when the Pi is booted up. It is only when power supply is connected to the Pi and the Pi is shut down that the sound occurs.
Not a big concern - just wondering if anyone else has experienced this?