Leonardo and micro: high pitch noise when powered from Vin
Yes, I've had this problem on a number of boards. It seems there are some faulty units coming out of the factory. I've had them replaced by my supplier, but I end up being sent more faulty boards.

It makes any kind of accurate ADC reading impossible.

A quick way to work out if your board has this problem


Place a jumper between 3.3v and A0 
Put an analogRead in a loop and println the result
When powered by 5V you'll see a fairly constant value (around 670), varying by 1 or 2
When powered by 12V, if you have a faulty board, you'll see varying values, deviating by 50 or more.


There's a couple of solutions/workarounds to the problem listed here: http://forum.arduino.cc/index.php?topic=235729.0.