2017 iMac got really loud

[imac275]

I have got a 2017 iMac (base but with SSD) that has suddenly become really loud with fan noise.

This coincided with a heatwave in the UK where the room with the iMac in would regularly hit 30C, so I thought it was understandable, but now those temperatures have gone right down, it is still doing it.

I'm not sure if the fan is at full speed, but it must be close as it has a bit of a ragged, uneven sound rather than just being louder.

It happens all the time the screen is active, even if there are no processes running.

Is this likely to be a hardware issue that I need to return it for, or is there something else to try?

 

[Djangoh]

I had the same problem with with a 2015 iMac. I took it to the genius bar for support, but they could not reproduce the problem when it mattered.
I recorded a video, complete with tissue paper strips taped to the top of the machine to show the insane amount of airflow. Simultaneously, I ran and recorded the activity monitor on the screen to let them them see that there there was no obvious reason for the fans to hit full gas. I don't think they ever really found out the cause, but I ended up with a replacement.

 

[niteflyr]

Try resetting the SMC.

https://support.apple.com/en-us/HT201295