G4 Tower with Accelerating Fan

[Natobasso]

I have a G4 Tower that has a fan that starts out loud on start up (normal) then goes to an acceptable level. However, after about 10 minutes the fan revs up to a VERY noticeable volume and I can't find any hardware power usage features in my system preferences.

System specs:
10.3.9 (just installed today with combo updater)
Dual 867 MHz Power PC G4 1MB L3 Cache per processor
512 MB DDR SDRAM

Any ideas on how to control the fan or make it cause less noise?

 

[Satcomer]

Have you cleaned it on the inside from dust lately?

 

[Natobasso]

Have you cleaned it on the inside from dust lately?

I guess that'd be a good place to start, duh.
(smacks forehead with hand)
The dust would make the fan work harder...