Unlikely to be caused by dust build up but that really depends on how bad the working environment is where your iMac Pro is located.
I have read on some third party Apple forums of users with iMacs located in machine shops having to use compressed air in aerosol cans to blast out dust created by the machine shop processes that was blocking the air intake ducts at the bottom of the screen on several iMac models. Care has to be taken if this is carried out on the iMac Pro models not to blast air into the speaker outlet vents which are alongside the air intakes as this might damage the speakers.
Putting aside the possible dust issue, as a user of the same iMac Pro model I have had several “thermal” issues in the four years since purchase from new.
The racing fans problem has happened several times when Big Sur was the OS and on a few occasions I found that on return to the Mac after sleeping overnight that the case and screen was very hot and the machine had shut down, on boot up after these events a warning message would state that the computer had been shut down to protect the CPU against overheating.
My iMac Pro has always been used in a clean home office environment and shows no sign of dust build up.
On one occasion I was re-installing the OS from a recovery boot up and the fans started to run at full speed while at the same time the Mac began pumping out a lot of heat and all that the recovery program was doing at that time was downloading the OS, it hadn’t even got as far as unpacking and preparing the OS for a reinstall.
I did a forced shut down and restarted the recovery - reinstall OS after the Mac had cooled down and this time it worked ok.
I have the impression that Apple did not sell many of these iMac Pro machines and therefore the experience and knowledge pool is quite small and since these fan/thermal issues are very sporadic there has not been much reported. I have access to two slightly older regular i7 iMac’s at work and have never seen similar issues with those.
All I can suggest is to carry out the SMC and NVRAM reset and if still on the Big Sur OS consider updating to Monterey, or, if you have already updated to Monterey, or are keeping an older OS for application compatibility reasons, consider booting into recovery mode and reinstall the OS anyway.
Since installing Monterey last year I have not noticed any high speed fan or overheat problems but these occur so randomly that this cannot be assumed to have fixed the issue on my machine.
Another approach might be to dive in to the Process Monitor the next time that the fans take off and see if any apps are using a huge proportion of the CPU capacity and closing that app to see if the fan speed drops back to normal.
I’m not sure that booting into Safe Mode would help with problem diagnosis in this case because the iMac runs so slowly in Safe Mode that the fans barely tick over anyway, although that might reveal a problem with the system control board if the fans do run at high speed when booted in Safe Mode.
If the problem is severe and continuous and the suggestions above do not help, or are not relevant, then the only realistic option is to book a service appointment with your local Apple store or independent Apple engineer.
Sorry, can’t think of anything else to suggest, hopefully somebody with relevant and recent experience of this problem will respond with a more appropriate solution.
