It this behavior from my Mac Pro fan normal?

[Moof1904]

While sitting idle, the fan on my Mac Pro (well, I think it's one of the fans) does this strange cycling thing. At first I thought it was a truck way off in the distance, starting and stopping. Then I realized it was my Mac Pro. Sitting with no apps running, one of the fans frequently cycles on an off audibly.

I recorded it. Have a listen. You have to listen past the low frequency rumble of the Mac and you'll hear the fan spinning up and then spinning down in cycles that are typically 8 or 10 seconds, sometimes shorter.

The sound file is hosted here.

 

[Makosuke]

It could be normal, but it's hard to say for sure.

I've heard several computer fans that will do more or less exactly that when the temperature the machine is running at is just at the threshold of where they start to ramp up. It could also be the fan failing to ramp up properly, or in theory even a faulty sensor, though I'd expect that to either keep the fan at minimum or full-blast all the time.

What happens if you are doing something that stresses the CPUs? Does it ramp up as expected? If so, then it's likely normal, though in general Apple's fan control algorithms are pretty good at not doing that. Could be that your ambient temperature just happens to be exactly where there's some hunting by the algorithm. I'd be interesting to know if it keeps doing it when the room is warmer or colder.

 

[Moof1904]

I've noticed it just the past few days. That's not to say that it started recently, only that I've noticed it. Running now with a bunch of apps open, the fans seem to be running at a constant level.

I've installed software to monitor the temp and RPM and have started a log to see if I can detect a temp/RPM range where this is reproducible.

I'll probably have some relevant data within a day.