Recently I noticed that there is a constant sound from my Powerbook 15' 1.33, it sounds like it is HD working, but when I give my PB some workload, I can hear actual HD sound which is a bit louder than that sound. What is strange about this sound - it is not the same all the time... It reminds the work of HD, though it must not be HD. What I'm afraid about is either failure or incorrect work of the cooler or HD. In both cases it may fail. I've tried to listen to other laptops (IBM & Toshiba), their coolers of HDs do not have such sounds. Does anyone else have a non constant very low volume sound background on his PB?