VERY noisy fan maybe?

HI all,

just wanted to find out if any one know of a way to kill the fan temporarily for diagnostic purposes My iMac is making a pretty bad noise. it started relatively subtle and has know become quick loud. I'm almost 90% sure it's the fan as it is loud and sound like the blades are tapping something as tapping /scraping something as they spin however the noise does stop and start again at irregular intervals. I 'm talking within 60 seconds it will be noisy for a second or to then stop for 5 seconds and then kick back in but even when it is stopped there is a constant whir. The noise is reminiscent of a read/write sound from a failing drive except it would be the loudest failing drive I've ever heard and there is no buzzing.

If I had a way to stop the fan for a minute I'd be able to isolate the problem.

The hard drive is functioning normally as far as read/write with no slowing down of the system.

Thoughts/suggestion?

Thanks in advance

J