New mobo Noise problem, Should i just get a new sound card?

[kpa2727]

i just bought less then a month ago a new asus mobo and built a whole new pc, the mobo is the Asus M3a78 - T Am2+ socket. it's onboard sound is great but heres my issue.

i submited a RMA tag for new egg but i don't wanna send out my mobo, so before i do. heres the issue i have bose companion 2 speakers linked up to my new pc build, all was good the first few days, i knocked out the audio jack , so i plugged it back in . all seemed well then when i shut the pc down i get ether a tone with a pulse sound from the computer speakers, or i get a high pitched whistle depending on if i shut the pc down with it plugged in or shut it down and plug in the speakers, it varries. now my question is , is this a bad power supply or mobo? i have tried other speakers and same deal so i ruled those out, oddly headphones are silent. the other odd thing once i turn the pc on it all stops. soon as i turn it off and shut it down it comes back, if i flip the power supplies main off switch obviously all sound quites, turn switch on and i get my noise back. so the question is what's the deal? i have a 850 watt thermatake power supply, i can replace that with ease but i really don't wanna pull the mobo out and wait 2 weeks for a replacement if i don't have to. also sound still plays fine when the pc is on and sounds great.

i posted this in the pc building forums to but please don't flame , i have only 5 days to get the mobo swapped if i have to, also i do very basic audio playing, sometimes i hook up surround but most of the time it's 2.1 channel, if you can please recomend a sound card from best buy.com since i work there and moneys tight , i'd be greatful but i think i might have a mobo or psu issue. i just don't know.

[not important]

Have you tried the speakers in another PC? Try the speakers plugged into a different power receptacle. If the speaker are good I would guess the MOBO is at fault. Something could have shorted when the audio jack was "knocked out" while the speakers were still powered on.

[kpa2727]

i tried them in the front port, the sound is fainter but there, the odd things i i noticed more issues linked to this, when the computer is turned off through windows everything shuts down but my usb plugs seem to get power, my usb powered portable drive still gets power and tries to be read, the sound of pulse back feed i hear is acctually the drive causeing distortion while attempting to be read, if i unplug it 90 percent of the sound goes away, the other thing is once the power button is pushed right as post starts the sound goes away. what's odd is why is the usb jacks getting power when the machines fully powered down?

[not important]

Can you power sown the USB potable drive? I'm thinking one of the USB powered devices may be causing your problem.

[kpa2727]

as i mentioned the drives stay running dues to the usb ports staying on which i really don't remember it doing before. and the drive causes the feeback as i also said, this only happens when the computers powered down.

[kpa2727]

i took the whole mobo out unpluged everything but the psu and audio , i still got the same sound so at this point all that's left is the psu and mobo. as crappy as asus's tech support is they said they think the mobos at fault to.