If your PowerBook G4 starts to make a weird fan variable vibration(purr-like instead of whirl/hum) I'd suggest looking at cleaning the intake vents and heatsink fan for dirt/dust/hair. I knew a few TiBook owners and while the design was prone to nasty oxidizing/corrosion if used near the ocean, they were built extremely well until the hinges failed but most PCs with similar external hinge designs suffered too. (I live near the ocean so owning or buying a used TiBook was just a bad idea, a friend from RI went through two AppleCare replacements a 500Mhz then a 667Mhz)
The only other possible noise would be if the backlight was going, notebooks/2000s LCD monitors would make a high pitch/mosquito buzzing noise with some mild flicker/slight uneven brightness before the light started to degrade/dim.
Based on the looks of that desk/computer environment too much electronic/electrical stuff is bound to create unwanted interference which can sometimes be heard on older notebook speakers--my old 12" PowerBook G4 was prone to a 60Hz hum or picking up a Quebec talk/music station during the winter while being a state away from the Canadian border--J'ai peur que je vais rouiller ce soir
