iMac start up problem - fan noise

When i try to turn my Intel Based iMac on, I get a loud fan noise that lasts for a couple of seconds. This just repeats all the time with a completely blank screen with no sign of life. The fan noise, sounds like when you try to wake the computer from 'Sleep Mode', only much louder...please help!
I can't afford an expensive repair bill, so I am praying for an easy fix.