G4noise.com - A site devoted to noisy G4's

What you could do is replace the fans with variable fan speed ones, and take temp. measures. Tweak the fans to the lowest point that your computer can handle, and that should help out a lot with the fan issue. Those things were just loud to begin with. Also you could drop some noise dampening material, though it could affect the airflow depending on how you do it. I know a lot of people replaced the P/S's with quieter ones and that kills a lot of the noise. There are a lot of options, though the forums over @ G4 noise are a trove. It's oudated now, but if you do anything, I would love to see a mod guide for what you do for our records.
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MacBook 2GHz Intel Core 2 Duo w/ 2GB DDR2 RAM & 120GB SATA 5400RPM HDD
Canon Rebel XTI
Google Cr-48 Beta Laptop