iMac Making Strange Noise

[TheHitcher]

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.

 

[fohawk17]

it might not only be a bad fan but also a bad power supply. it might not be cutting power to the fan. the only thing it is supposed to power after shutdown is the mother board so it can keep time settings and such

 

[Macsleuth]

Screen Brightness

I noticed a similar sond when I turn my screen brightness down.
ry turning your brightness all the way up.
Then play with the fan speed and see if that changes it at all.