Desktop computer case fan slowing way down at intervals - cause?

I have a Cooler Master case with several fans on a custom-built PC I’ve been using for years.

Recently one of the fans on the top of the case started making a loudish noise every 5-6 seconds. I got a flashlight and watched it while running and realized the fan is spinning way down and then spinning back up again to a faster speed, and that is what is making the noise.

Strangely, I moved the fan’s connection from one labeled “CHA_FAN” to “PWR_FAN” (the only free slot), and it behaved slightly differently. It is running on a lower/slower speed mostly (and not making the noise), but occasionally, it still spins up to a faster speed and makes the same noise when it slows back down.

I could replace the fan of course, and I’m hoping that’s all I have to do, but I want to make sure it’s not something else, like an issue with the mobo’s connections (mobo is an ASUS Sabertooth X58). I also remember in the past trying some (possibly 3rd-party) software that was supposed to monitor/regulate fan speed but I don’t think it worked as intended.

I’m probably going to try a few more experiments with hooking different fans to different mobo slots (and looking at BIOS settings - there are some for fans right?) in order to further isolate the issue but I wanted to see if anyone had experience with this kind of thing / ideas about what the issue most likely is. Thanks in advance for any advice.

A lot of PCs control the fan speeds based on temp sensors inside the case or on the motherboard. Maybe the inside temp is getting a little hot at times or the sensor isn’t working correctly. I doubt that it’s the fan itself. Have you opened up the case and blown out all the dust bunnies lately?

Yeah, I just did that when I changed the fan connection (what’s the difference between CHA_FAN and PWR_FAN anyway?), so that might be why it’s staying on a lower speed more now instead. The frequently oscillating speed still seemed a bit strange to me.

“CHA_FAN” is probably for the rear case fan. “PWR_FAN” is usually there for power supplies which have a cable run out so the motherboard can monitor its fan speed. I think in most cases the power supply still powers the fan itself though.