I just got a used gg5 for my studio, and it's not incredibly noisy, but considerably louder than my macbook pro. . . should this be happening? any tips on how to reduce computer noise or work around it?

If you don't have an LCD get one. That will quiet things down considerably.

Is this a powermac G5 tower? mine is very quiet, much quieter than the macbooks friends have which get way noisy when working hard

BUT when i added a faster graphics card with a fan it became noisy, had to get a quieter fan for the graphics card, althpugh the extra heat in the system still made the mac fans run faster too. does yours have a graphics card fan? if so thats often a big part of the problem, get a 3rd party quiet GPU cooler from a company like zalman. If you dont have a graphics card fan then i would imagine your G5 should be pretty quiet...

nevertheless despite mine being fairly quiet it is now in a wooden cabinet, with a 120mm silent fan on the inflow and another on the outlet of the box, rear mounted so i hear them even less, and a baffle inside the box so that the airflow goes through rather than backing up inside the box back to the computers inlet.

Argeed with Tom
But sometimes fans make it nosier. Open the computer is see if its of the fans that are making the nosie or check the power supply

Besides the above, open that puppy up, could have a fair amount of dust on the fan blades making them noisier and less efficient.

yeah, something dusts on the fan blades will make them noisier and less efficient.

The 1st thing you'll have to do is identify which fan is the noisy one, then take it from there. Make sure to remove all the dust accumulated on the heatsinks too. Do NOT use a vaccuum!

another reason is your hdd. maybe they keep spinning faster and that why noise sound come out.