iMac Maintenance

Increased fan noise where it was not noticeable before can be from increased fan speed, a normal programmed response to the detection of elevated temperatures inside the iMac case.

Higher temps can be due maintenance, workstation layout, or software.

Maintenance: First, DO NOT attempt to open the iMac case! Simply make sure cooling air can get in and out of the computer.

The air intakes must not be blocked by dust/lint, or stuff you stacked under the computer monitor. Intake vents are on the bottom edge of the case:

Do not use "canned air" to clean the vents. That can blow the dust deeper into a computer that Apple did not design to be opened by users and, improperly used, can induce moisture inside the system.

I place my iMac face-down on a soft towel and hold a vacuum nozzle about two inches from the vents and use a non-static brush to flick the dust toward the vacuum nozzle without letting the nozzle touch the computer case. I also work quickly; any static electricity generated builds over time.

Workstation Layout: The owner of this iMac came here complaining of serious overheating:

Removing his "library" from under the vents fixed his problem in minutes. Put NOTHING between the vents and your desk surface.

Another documented workstation issue is having the monitor above eye level, requiring it to be tipped fully forward. Had I not duplicated the problem myself with my iMac 27, I would not have believed the post here that first reported it.

The important hot air exhaust vent in located above the RAM door and below the top of the iMac stand:

When the monitor is tipped forward, the stand is 1/4- to 1/2-inch from the exhaust vent. That slows the exit of hot air and temps go up. Like I said, been there—done that.

Software: Anti-virus and so-called "cleaning" apps WILL increase the workload and thus the temps. Do not use! Remember that Macs—catlike—clean themselves. For over 20 years the macOS has featured elegant, automated self-maintenance routines that run in the wee hours of the morning.

These do ALL the housekeeping your iMac needs, even defragging. Install NO third-party apps that claim to "clean, protect, optimize, or tune." They interfere with the elegance you paid Apple to build into the OS and will cause issues you won't like.

Some apps are natural-born heaters. Streaming video and video-conferenceing work but the CPU and GPU hard and temps will rise. Same with most modern games.That is why your iMac has a thermostatically-controlled cooling system.

You can run the Maintenance scripts for the Mac with this Terminal command:

sudo periodic daily weekly monthly

followed by the Return key. You'll be asked for your Administrative password but it will not display in the Terminal window.

👍+1 Vacuuming the intake vents should be your first step. It may be all you need to do.

My iMac started acting up a few years ago (display problems, not fan noise). After getting all kinds of "advice" about problems with the GPU, display inverter, macOS and more, the problem turned out to be nothing more than overheating because the intake vents were clogged with dust! After some generous vacuuming all was fixed and I haven't had a problem since!

Restart in Safe Mode and then restart normally and test.