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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?

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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)

  • So could it be that the noise is directly related to power consumption (which is then directly related to CPU usage)? – Szabolcs Aug 18 '14 at 23:22
  • Yeah, I should think that power circuits are most likely to be the source, due to the relatively low operating frequencies. I should add that not all motherboards exhibit this since small variations in the way components are attached can change things, but it's generally harmless. Looking some more, I see some claims that failing electrolytic capacitors can make a high pitched sound, but nothing about it in reliable publications. – Michael D. M. Dryden Aug 19 '14 at 0:40
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I had annoying 'access' chirps, and thought it was excessive CPU usage. Which it seemed to react towards... fired up disk utility (SSD disk) and ran all the checks, and fixed permissions. Chirp gone...

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