Okay, let me take a stab at this and give you some ideas.

As I understand it, this is a physical noise from the computer and not 
from the speakers, right? If it IS from the speakers, it could be 
related to a bad ground on one of the many peripherals that are pluged 
into the AC outlet strip. A bad ground can cause a ground loop current 
to occur - which is not healthy for anything. Since the software is 
acting erratic when the noise occurs, it is affecting the proper loading 
of the software, which will mess it up. All three wire plugs should have 
less than 0.2 ohms resistance between the ground pin and a metal part of 
the device.

You have disconnected all CD-rom devices and any spare hard drives and 
just have your boot hard drive connected. (While doing that, check each 
power plug (four pin plug) for any pins that are pushed back. A faulty 
connection can cause lots of trouble. Be sure to eliminate any other 
peripherials you don't need for checking out this problem (printers, 
scanners, etc.). You want a bare system (keyboard, mouse, monitor).

You stated you swapped out the PSU with a KNOWN good one. Make sure it 
is a known good one. Is it a new one? If it is an old one that was 
working in another computer without any problems, it MAY be a good one. 
Since your problem is intermittant - you really have to be careful with 
what you swap in.

On the motherboard, you may find a small, black round object, about 1/2 
inches in diameter, with a tiny (1/16th - 1/32nd in.) hole in the 
center. This is a speaker. Some motherboards have them, some do not. Is 
the sound coming from the speaker? Test it during a noisy boot by 
putting your finger over the hole. If it deadens the sound, then it is 
something in the software/board that is causing the PC to generate the 
noise.

Capacitors were mentioned. While a motherboard make look good, there 
were problems with Electrolytic caps made since about 1999 (some cap 
manufacturers stole a formula for electrolyte paste from a company and 
used it to make their caps. Problem: formula stolen was incomplete - 
therefore the electrolyte they made was bad, and large size Aluminum 
Electrolytic Capacitors in many devices (PC Motherboards, PSU's, etc.) 
were failing after about three years of use. Your computer may be 
outside of the problem years, but just in case, look at the large size 
caps (usually around the CPU) to see if they are bulging at the top or 
leaking at the bottom (a brownish fluid or whitish powder). If so, you 
have a bad cap problem (see www.badcaps.net). It may be contributing to 
a messup of the software loading which in turn could be causing the 
noise problem.

Do you have a battery operated toy recorder with a microphone (the kind 
kids sing into - like a fischer-price or some other brand)? When you 
boot the computer, with the recorder going, run the microphone in a 
gride pattern around your computer (cover off) and as you go into 
certain areas you can make a verbal clue (shout A, B C) according to 
your own plan. When you play it back, listen for an increase in the 
noise sound and relate it to where the microphone was (between C & D). 
That should help you pinpoint the source. A toy recorder usually has a 
plastic case microphone so you can get really close to things without 
worrying about shorting anything out.

And yes, the monitor can cause a problem an feed it back into the 
computer. The electronics can handle some electrical noise, but not when 
something faults and creates current loops.

Hope these ideas help.

Jean Bourvic

Dan Deucalion wrote:

>On Sun, 26 Feb 2006 15:26:50 -0500, Chris Ryan <[log in to unmask]> wrote:
>
>  
>
>>There's still one more moving part, the PSU fan. Can you briefly stop it
>>    
>>
>>from spinning (say, with a toothpick?) at bootup just long enough to rule
>  
>
>>that out? Otherwise I'm thinking a capacitor on the MB is starting to act
>>up. Have you left the case open and tried to listen during boot up to see if
>>you can pinpoint the noise?
>>
>>    
>>
>
>I swapped out the power supply so I know that wasn't the cause.  I remove the cover every time I boot up to try to zero in on the noise location.  So far, no luck.  How would I be able to tell if a capacitor is faulty ... and would it be possible for the problem only to exist for 90 seconds or so at boot up?
>
>Dan
>
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