iMac Making Strange Noise

Hi all, i've got a small but annoying problem.

My 24" iMac 3.06GHz is making a strange noise when it's on.

It's like a low pitch buzzing that sounds like it's coming from behind where the Apple logo is on the front. I think it's the "ODD" fan (I'm not too sure what that fan actually does) because i installed smcFanControll and if i set the ODD fan speed faster the noise goes higher pitch.

But the really annoying thing is, when i shut the iMac down it carries on but lower pitch, like the fan is slowing down but won't completely stop and only stops when i pull out the power cable.

Do any of you have this problem? or have you got any idea what i can do to fix it?.

Thanks in advance.