[smileLP]

when i start a new application, or perform some other function slightly more demanding than average, it sounds like a helicopter taking off. do i have a defective unit? (the computer, not my...) or do they all sound like this? thanks

[Nobody]

Mine is as quiet as a mouse, are you in Europe per chance? I have heard of a lot of complaints from Places where the power supply is 220 v or 240 v, apparently the power-supply chirps etc.

[Grant_G]

Mine revs up occasionally, usually just for a moment while it decides if it really needs it (Apple errs on the side of caution, so if the system just suspects heavy CPU usage, the fans come on). I have just two apps that make some of the fans run full-time. Both are Classic apps, and both are ported from the Wintel world.
G

[Argent]

Same here my G5 revs up for a few seconds once in a while usually when it's doing something that's particularly processor-intensive.

[JohnVL]

Some of the earlier 10.3.x systems were not as quiet as that later ones. If there's a continuous hum it could be a power supply.
Is it really dusty inside?
Have you run the hardware test found on the second system disk?
Otherwise I'd repair permissions, run Disk Warrior and/or Tech Tools Pro and see what happens.
Good luck,
John VL