intermittent fluttering noise from iMac

What size and year model iMac?

see > Identify your iMac model - Apple Support

Shutdown, unplug, check and clean (vacuum) the air intake on the bottom, plus the intake and vent on the back.

If it is older and the intakes/vents are real dusty, then might be best to schedule a service visit for full cleaning.

If it's an older iMac's with spinning HDD, then chattering, fluttering or clicking might suggest eminent HDD failure.

If that is the case, then it would wise to backup and schedule a service visit for evaluation and replacement.

Again: That is starting to sound more like hard drive noise than fan noise and I recommend that you schedule a service appointment for evaluation. see > Find Locations

You're welcome.

Is this a 21.5 or 27 inch iMac?

That is starting to sound more like hard drive noise than fan noise and I recommend that you schedule a service appointment for evaluation. see > Find Locations