fan huumming as loud as jet

[pujan14]

My hardware conf is

Machine Name: iMac G5
Machine Model: PowerMac8,2
CPU Type: PowerPC G5 (3.0)
Number Of CPUs: 1
CPU Speed: 1.8 GHz
L2 Cache (per CPU): 512 KB
Memory: 512 MB
Bus Speed: 600 MHz
Boot ROM Version: 5.2.5f1

now as soon as i start mac in just few seconds it starts fan on top speed which makes humming sound quite loud what should i do to stop this

 

[harryb2448]

See if resetting the SMU is any help in controlling the fans:-

http://support.apple.com/kb/HT1767?viewlocale=en_US

 

[kermada]

Check if there is a large amount of dust at the bottom.
I think that is what killed my mother board.