m4 ATX usb sound card ALOT OF NOISE

so here's how it is: i have an install wich uses the m4-atx power supply, and for sound i got an external usb car (creative x-fi). untill recently it worked prety good.
BUT, now i face th following problem:
the one chanel of the inverter (only the one). causes ALLOT OF noise. continiously, with lesser periods of sound (like a bass or something). whenever i play sound from pc or not.
- the inverter works fine with other input (mobile)
-the sound card works fine on other pc
- the sound starts, as soon as the metal of the usb cable, of the sound card, touches the mb (no need to connect)
-i dont know what causes the noise to raise, but when it was big enough, the sound card was getting disconected-and reconected all the time...

what i did for this to start is this :
i bought a new video card (noise continius even without it connected)
i placed enother LCD (noise continious without)
and i moved the whole pc install. i have 2 batteries. before it was taking voltage from the one, and now from the other. i have tried many diferent grounds but no luck..
needles to mention, that the LCD'S also have allot of noise in the image!!
any help ? :D

If you're having a lot of noise in the image and in the sound card it leads me to believe that you've got the power cables running down the side of the car too close to the VGA/Sound cables. Maybe try running the image and sound cables on the opposite side of the car?

nahh, they are arledy at the opposite.. and dont forget that the noise, begins as soon as the metal of the usb input of the sound card, touches the MB!!

have you hooked the usb sound card temporarily to a laptop just to make sure it's not actually the sound card? or tried the sound card with a boombox or other powered speakers instead of your car amp?

maybe a floating ground issue. you could try grounding the metal jacket of the usb to the car chasis. i don't know what amp you use, but with most it's safe to ground the negative of the inputs (the outside of the RCA connector).

"one chanel of the inverter" - what inverter are you talking about?

what does the noise sound like - hiss, whine, popping?

well some updated...
i wnet and bought a new motherboard, only to return and find out that the PSU, had broken down. the light flashes, and it turns constant if u try to power up the pc (meaning it is working), but no voltage on its rails!!
then i went and bought a 600watt inverter to use with a normal pc psu. the inverter worked fine at the start, but soon after it also broke down.. the light turns on (As if it would work) , but no voltage oupout... (17 instead of 220).. so i am starting to think that both got fried cause of something else. the Battery on wictch i have them connected , is a really old one(i use two batteries), and ONLY in the case of the inverter, it as connected to the possitive of the old battery, and in the ground of the new one!.. any correlation?