Wind coming from desktop Mac

Check the air vents on the underside (bottom edge) of your iMac. Over time they accumulate dust and at some point it restricts airflow enough that the fans run faster. Use your vacuum cleaner's hose to pull the dust from the vents (btw, the vents are tiny and they are along the entire bottom edge of the iMac).

Just vacuuming along the bottom of the iMac is sufficient in the vast majority of cases. It's easy to do and if it resolves the problem there is no need to go further.

If that plus a full restart does not resolve the problem then I'd next consider a runaway app or process and use activity monitor to try to identify it.

Attempting to remove the display assembly is a delicate procedure with great risk of damaging it in the process.

Your workstation can affect the air intakes. Even with clean vents, temps will go up. This image was posted here a few years back where the user reported severe overheating symptoms. Note the "space" between the work surface and the air intakes:

When I worked in law enforcement, we called that a "clue!" iMacs need that ~3 inches of clearance.

Also check the air exhaust vent. It is under the stand in the middle of the back panel.

Avoid a bit of bad Apple design re that vent. If you tip the screen fully forward, the "neck" of the stand gets too close to the exhaust vent and reduces its efficiency. Someone reported it here, but I did not believe that could happen until I tested on my iMac. Sure enough, when the screen forward and the vent "pinched," my fans sped up and an aftermarket temps app showed the CPU temps going up fast.

...I've rarely used for the last several years.

Was it plugged in the entire time? If not, the internal backup battery—intended to maintain settings for short periods of service or storage but not months—will run down and the computer will forget important things.

I agree with the other comments; I'd first go with the SMC reset already mentioned.

First try restarting the iMac.

Then if that doesn’t help, reset the SMC.

Reset the SMC of your Mac - Apple Support

you should be able to inspect the long rectangular intake vents running along the full width bottom edge (right below the Apple logo on intel iMac)

i would suggest laying the iMac on its back, shining a light into the vents, and looking a couple inches deep for plastic grids -- the holes in these grids need to be free of dust -- if they (or more internal dust) are impeding proper air flow (you cannot see deeper into the iMac)

you should remove the screen assembly and blow out the machine

with the screen removed -- this would be a good time to replace the old HDD with a SATA SSD -- because the HDD is painfully slow compared to SSD and the HDD is likely worn out

a shop charge for this service would be minimal

i would suggest a higher end SATA SSD like Samsung 870 EVO or 860 Pro (don't cheap out internal parts)

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however, this is not to say a clogged internal air flow is causing your fans to run -- sometimes the fans run (Activity Monitor may provide a clue) -- you do not say you have restarted the machine, that's where i would start...

Try running the Apple Diagnostics to see if any hardware issues are detected.

A temperature sensor which is no longer communicating or is sending a value out of range can trigger the fans to run at high speed as a protective measure so that the system does not overheat since the system doesn't know how hot that particular section is getting if the sensor is bad.

That would be the cooling fan working overtime.

removing an iMac screen assembly is not: DOOM and GLOOM

it's routine maintenance (probably 40 minutes labor start to finish) -- and the shop is responsible for any damage

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swapping SATA drives is probably all of 10 minutes (after the screen comes off)

i imagine one could get both tasks done for a hour's labor

shop rates vary $100-$300 per hour

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pulling out the main board to swap NVMe SSD, replace RAM (on 4k models), and the battery -- now that's hairy for most people -- but routine work for qualified tech (probably 2-3 hours labor)

Good advise MartinR. However the iMac should be shutdown and unplugged, before any vacuuming or cleaning is done.

den.thed wrote:

Good advise MartinR. However the iMac should be shutdown and unplugged, before any vacuuming or cleaning is done.

Yes, absolutely.

You're welcome.