Thread: Car pc noise Not alternator noise. Please help

Well I finaly got to installing my car pc and I unfortunately have buzzing noise and almost screetching noise coming from the speakers at hight volume during music pauses or when music starts fading out at the end it is alot more noticeable. It changes with volume it does not change whith engine rpm so I know it is not alternator noise. It seems i am getting some interference from my hard dfive or other things int the computer. It seems like when the computer is proceessing information you can hear the fax sounding screetches.
I have a Mobile computer solutions Car pc
I am using Soundblaster xfi 5.1 as my external usb soundcard
It is connected to my Pioneer Avic N4 as an AV input
I am runing 2 alpine PDX amps 1000.1 for my subs and 600.4 for my speakers.
I have no noise when I am using any other functions of the avic like sirius/xm/ipod/ radio. I know my system is noise free to start with I only have an issue with my pc.
I dont know how I am getting this noise. Is it induction from PC motherboard/hard drive ect...
The soundcard is connected by USB to the computer so I thing the sound should be in a digital form until it is converted to analog RCA through the sound card and somehow that is where my noise is being created.
I am not too car pc saavy but has anyone encoutered this issue or has a suggestion.
Thank you