[SOLVED] Help me diagnose a hum

[darsu]

I'm being slowly and steadily driven crazy by a persistent hum from my computer. I need to figure out which component causes it so that I'll know what to replace. The problem is that I can't pinpoint the source: the hum is only audible when I'm at least a foot away from the computer and if I try to close in on it I can no longer hear the sound.

The culprit has to be either the power supply, the CPU fan, the hard drive (there are no other moving components) or something in the case acoustics. The hum is a slightly sharp C#, I believe the one a minor ninth above piano middle C, which would mean the frequency is somewhere around 560 hz. I would compare the sound to a sustained flute tone. What do you reckon it is?

The sound is quiet enough that I wouldn't mind it if it was just a tuneless whirr, but since it's a clear note it's driving me up the wall.

[pp.]

I take it you mean a major prime above c?

Have you considered insulating your machin from the furniture? Also, there might be a standing wave in your room. If the sound is really as well defined and of narrow bandwidth as you say, it might be worthwhile to compute the lambda and go looking for columns of a half and/or a fourth of that dimension.

[squaregoldfish]

This most likely to be a fan problem, so let's start with that.

The best way to diagnose which fan it is would be to temporarily stop each one in turn but putting your finger on the middle (don't stick your finger in the blades - you'll get blood on your components!). Since you can't hear the noise close up, you could get an accomplice to stand in the appropriate position while you stop each fan in turn. If none of them stop the noise, it's probably the power supply (you can't stop that fan safely).

If you can't get an accomplice to help, and you can hear the noise from the moment you switch on the machine, you can get away with unplugging the fans completely as long as you don't run the machine for more than a few seconds. (I'm confident you can do this even with the CPU fan, but I won't take any responsibility for it - all I can say is that I've done it myself.)

To check the hard drive, you could try unplugging it and switching the machine on. It won't boot, but you might get rid of the noise.

When (if?) you find the source of the problem, it's either that the component is faulty or it's causing vibration. It's worth removing the component and re-seating it, making sure there's a good tight fit and all the screws etc. are firm.

Steve.

PS OK, so mine is a little less complex than the above. But it might work. :)

[will1911a1]

Last time my rig was making an odd noice it went away after I gave the case a couple whacks on the side.

It would seem to me that it it were something with a fan the noise would get louder the closer you got, not go away and come back when you move away from the case.

[darsu]

I put my finger lightly on the fan center to slow it down, and the tone got lower, so that settles that. Thanks for such quick replies, everyone.

(...how about "augmented octave"?)