i Mac fans hissing at start up

What year model is this iMac?

see > Identify your iMac model - Apple Support

Have you tried resetting the iMac's SMC?

see > Reset the SMC of your Mac - Apple Support

Has any hardware ever been serviced, replaced or upgraded?

(a broken or disconnected temp. sensor can cause the fan(s) to run out of control)

Try resetting the SMC up to 3 or 4 times in a row.

Reset the SMC of your Mac - Apple Support

Do they stop soon after powering up?

Your air vents may be clogged; how old is the machine?

What's the fan speed? I use the app Fanny to display that. The fans run all the time; my 2020 iMac fan is running at 1200 rpm right now.