Yeah, I had a similar thing on my 867. Just a very quiet noise, but it would come and go - either that or I just didn't notice it all the time. My solution : Apple Pro Speakers and iSub! 
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Home Kit
MacBookPro C2D/2.16 2048Mb 120Gb OS10.5
PowerMac G4/867 1152Mb 440Gb OS10.4.10 ProSpeakers iSub USB2.0 S-ATA BT DVD±RW Zip250!
Custom Black&Platinum PowerBook G3/300 384Mb 40Gb OS10.2.8 FW
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