Acer desktop making whistle sound

I have a Acer Aspire desktop model AST690-UP626A that is making a whistle whenever turned on.  From reading on the web I'm thinking it is the power supply a FPS Group Inc model ATX-250PA Part no 9PA250BF00.
Leaving the 20+4 pin connector plugged in, everything seems fine.  However when I plug the P4-MB connector in the whistle starts.
I need to know if this could be a motherboard issue or power supply before I go swapping parts on the customer.
Heath CalhounAsked:
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Did you check the power supply fan to see if a blade is chipped or anything?  Does it sound like a system whistle (beep or tone) or an air flow whistle?
Does your machine Boots till OS ?
Heath CalhounAuthor Commented:
1) checked fans, fans intact and do not make noise when spin manualy. sound does not generate when P4-MB unplugged.  Returns soon as I plug this back in.  Fans also spin normal with P4-MB unplugged.

2) No, the system will not even POST.
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I guessing that if that plug is not connected and the whistiling doesn'^t happen, it is probably not the power supply, but whatever that plug is powering.
Might be on the main board, or some extension card (graphics, Lan, etc.)
Try removing all extension cards (pci, agp) and boot again.
If you have multiple ram modules, remove all but one, try again, if whistle still happens, swap it for another.
this way you ruled everything out except the mainboard and the power supply.

It might be hard to find which one of these two it is. But maybe you have an old spare power supply laying around to try?

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Heath CalhounAuthor Commented:
only power supply i've got has a 20 pin and 4 pin not a 20+4 would it still work though?

everything is built into the motherboard and only 1 stick of memory.  don't have a mate to swap with it.

i have not yet tried to post the system with the P4-MB 4 pin unplugged.

also all the caps on the mother board appear to be good.  none popped or looking like they are about to.
Might it also rather be a sound similar to a siren? If that is the case, it usually points to an overheating CPU. Remove the Heatsink from the CPU, make sure both surfaces are cleaned blank and shiny. Also make sure the CPU sits properly in it's socket. Then add a tiny drop of fresh thermal transfer paste to the top of the CPU and firmly and properly re-attach the heatsink.
That whistling could be a capacitor:

It's time to replace the motherboard, or if you're handy with a soldering iron, replace the capacitors.
AFAIK, you need to replace one or the other, PS or mobo, to be sure which one is bad
note that BOTH can be bad (a bad PS can kill a mobo, and VV)
best look around for a PS first - less cost, and easier to change -
maybe a friend has a PS that you cna borrow -  or have the pS tested in a shop, or with an OLDER pc
MikeIT ProfessionalCommented:
I've requested that this question be deleted for the following reason:

This question has been classified as abandoned and is closed as part of the Cleanup Program. See the recommendation for more details.
Heath CalhounAuthor Commented:
turns out to have been motherboard acording to a tech store.
tried a different power supply did same thing. whistle started when plugged
p4 mb connector in.
Heath CalhounAuthor Commented:
I have reopened your question and started the process of closing it with a split amongst two or more Experts, as that outcome seems more equitable than your original disposition.
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