Arduino Micro Humming (screeching?)

I'm soldering up a board and when I connected power, the Arduino Micro started humming. It's a high pitched whine, it doesn't happen when I connect power via UBS, but it does when I externally power the board.

I have the Vin pin connected to a 12V line, and that causes the noise. It's not pulling big currents or anything, and it seems to work fine, but the noise worries me.

Anyone else had this issue? It may not even be an issue, just a curiosity.

More likely the noise is coming from the 12v supply - but it could be a cap on the board too actually making the noise. As long as the 5v is clean, you're fine.

Ceramic caps change size ever so slightly when charged vs discharged; if this is happening at the right (wrong?) frequency, you get "singing capacitors". Regardless of which board it's on, this is what's happening. The frequency and volume will change with the load.

Thank you!

I read a thread about a Leonardo humming. Apparently it was the cap right next to the barrel jack.