[G4] Noisy G4/933 Quicksilver

[Pat Crowe]

When my G4 is first switched on, it is reasonably quiet but after 20  
or 30 minutes the noise level has risen considerably. I understand  
that two 7200rpm HDs are going to make some noise and I am assuming  
this is the noise I hear when first starting.
If the extra noise is being generated by the cooling fans, can I  
assume these cooling fans are driven by demand? ie. as the  
temperature climbs, the fans start and then increase speed as required?
I note that the air exhausting from the CPU is not even warm.
Jessamine.