R2D2 Mac Pro making sound when CPU usage is high (not fan noise) We have a 12-core R2D2 style Mac Pro at the office.  It is very quiet.  If I put my ear very close to it, I can hear the fan humming smoothly, but from a distance it's quiet.

When I run a program that uses all 12 cores at 100%, the computer will emit a strange zzzzz sound comparable to a creaky wheel.  The sound is very quiet, but still audible in an otherwise quiet room.  It's slightly louder than the humming of the fan, and it is not smooth like the fan.

It doens't sound like this is the fan spinning up: the sound comes on isntantaneously as the CPU usage goes to 100%, and it stops instantaneously when I stop the program.  I can still hear the steady hum of the fan in the background.

Question 1: Since this computer doesn't have any other mechanical parts than the fan, what is making the sound?  Is it an electronic sound?

Question 2: Does the sound indicate a problem?
It's normal. Many computer components can generate high frequency noise under high load, mostly from power circuits, but fans are usually loud enough to cover them up. The GTX 780 in my desktop does this but I didn't notice it until I installed a liquid cooling system. If you're interested, this has some details of the components that can cause this kind of noise. (one example is piezoelectric flexing of ceramic capacitors causing vibration of the PCB)