Yesterday, my PBG4 malfunctioned. It was playing CD DIGITAL AUDIO (all built-in hardware: drive, DAC, amp-speakers), when it suddenly started emitting a loud noise. At first I would have described it as gunfire popping. It was loud and continuous. Desired CD audio stopped sounding, it was "replaced" with this loud noise, I did not try muting or changing volume.
Whole
machine seemed to still be behaving (aside from the noise), drive was still spinning CD, trackpad and keyboard worked, OS did not hang-stall-freeze, I opened Activity Monitor (or whatever that .app was called) and found nothing unusual. I know OS and HDD were behaving because WhateverMonitor.app was not cached in memory. I chose to eject CD and actioned Shut Down.
I disconnected power, turned over, felt all around. PowerBook was mildly warm. I highly doubt it was overheating then. I removed twist-tie, reconnected power, turned on. Pop-gun noise still there, no POST "bong", Mac OS X started in safe mode. I checked System Profiler diagnostics category, it said POST passed. I actioned Restart.
Now Mac is at point of no return. It makes pop-gun noise lasting only two or three seconds (around when POST "bong" would sound?), then quiet (aside from fan, which is as loud as a vacuum cleaner). Display and backlight stay off. I hold power button to power off, then power on again, same symptoms. I power on and depress keys to reset PRAM, this seems to be doing something: after an appropriate while (about as long as PRAM reset would normally be) I hear noise again. I keep keys depressed: repeat wait time to noise, then repeat. So maybe this unpleasant noise is instead of POST sound.
Does PowerBook G4 have a POST fail sound?
I close PowerBook and put it away, then search WWW for PowerBook gunning noise, PowerBook popping noise, find nothing relevant. Then I try to think of other words to describe it, think of it sounds a lot like
engine braking, so I search some more, find nothing relevant. I can record this sound if anybody is interested, but I tell you, it sounds just like truck-tractor engine brake (or some call it "jake brake").
I did find a case of different malfunctioned PowerBook noise: game show buzzer (stupidtube link).