I donīt know if I can help you but I try.
Concerning the low noise:
If its always there, even if the machine is turned off, it might
interfere with some other cable lying there..?
If the frequency changes with the rpm of your machine its
probably a grounding problem. -> Go directly to the battery, use
thicker power cables, high-quality connectors..?
Perhaps you could also try to get shielded rca-cables.
I didnīt hear your noise so I canīt tell you if THIS is normal, but a
small noise is ALWAYS there.
Getting a better soundcard might also help. Try SB64Gold. Old,
but still fine. You can get them on ebay for cheap, but its ISA.
PS. Disable the mic, the line input and everything that you donīt
need in the controlpanel.
Hope I could help you...