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?