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John Siracusa at ArsTechnica points out a bug I’ve experienced more than once with various Macs. High frequency changes in CPU power draws on some Macs’ power supplies can cause them to emit sounds.

Then I noticed a strange noise…a chirping sound. I chased down the source, I searched the net for solutions, and I eventually wrote about it here at Ars in March of 2004. The summary: the power supply in the revision 1 Power Mac G5 made chirping noises, and there was no hardware-based fix in sight.

Sure, it’s hardly an OS crippling bug, but in a quiet environment it can be quite distracting. This bug also affects more than just G5 hardware – I can hear my PowerBook G4 (1.25 GHz 15”) making sounds at I type. I’m positive of the source for two reasons. For one, I use MenuMeters to display a small CPU usage graph and ATA bus read/write indicators in my Mac’s menu bar. Neither are active, yet I can still hear the faint clicks. Second, John links to a utility called SystemLoad which will actually play a scale on the power supply by adjusting CPU usage levels. Hearing my machine’s power circuitry play a tune is, well, creepy and unsettling.

John goes on to say that a feature of the processors, CPU “napping,” allows the unit to temporarily lower its power consumption and “wake up” as needed, often many times a second. I’ve found that this feature can be controlled by installing Apple’s CHUD (Computer Hardware Understanding Development) tools and disabling the napping option in the new Processor system preference pane. (As a side note, the Processor prefpane icon changed from a Motorola chip to that of an Intel Pentium 4 style chip many months before the Intel transition. At the time I wondered why they made the change, but only recently has the tiny interface tweak become clear.) Upon unchecking the box, the change is immediate and the clicks are silenced. SystemLoad no longer plays tunes with my analog hardware. With napping disabled, though, the CPU tends to generate more heat. In return, the fans come on more often and kick up even more noise than the faint clicking. This situation is lose-lose, and is one I should not even be dealing with considering the original selling price of the machine. Apple needs to get with the program and start producing quieter analog electronics. I know digital is all the rage these days, but electricity isn’t going anywhere for a long, long time. Or, more accurately, it’s going everywhere all the time, but it should not make noise doing so.