Noise and everthing else
A system can produce unessasary noise due to various reason ranging from CPU fans to loose lines. Now there is no standard procedure to go around this problem only a few tips that can help you out.

Mixer settings
Try to mute and reduce channel settings of the individual channels in you mixer individually (in my case im using the ALSA mixer). The hiss or some kind of disturbance can be caused by one of these channels being active when they are not required. A lot of times 'Tone' or the '3D' channel in your mixer are causing the trouble . 'Tone' channel in the alsamixer added to a lot of noise(hiss) with soundblaster Audigy2 soundcard. Following are the channels which I found out were supposed to be active in the alsamixer while we recorded on Ardour through the 'mic in' of the audigy2 (external interface).

<master> 80%
<PCM> .... which is digital audio
<Line 2>
<Line2 Ca>  ....this is Line2 Capture
<Aux2 Ca>
<EMU 10K>
<Front>  

Setting Capture Source (Channel)
 Now in some sound cards we have to set the required channel as the capture source. this can be done by using alsamixer and going to the required channel and pressing the Space bar this would activate capture on the required channel. For eg we need to set <line in > as the capture source we get to the <line in> channel using the arrow key and press the space bar this would activate capture on <line in>
and check out this link for what each of the channels represent on the ASLA mixer on audigy2 soundcard.

No Halt Option
If you are getting the noise which corresponds to system activity like mouse movement, Disk activty, System idle time)then, Boot the Kernel with 'no-hlt' option. A lot of times when the CPU goes into the halt then it dirties the power supply a lot which is not very good when recording and a lot of sound people dislike it.

Cables and Adapters
make shure all your cables are connected properly to whatever point. There are no open jack ends lying around, this could end up adding to the noise. Use better quality shielded cables, this can reduce the chances of noise. All cables including internal cables(ie inside the computer eg CD to sound card) and all the external cables connecting the mic, mixer and amplifiers should be of good quality and well shielded. Try to keep all the cables inside the computer away from the sound card. Try minimise the use of adapters and use only good quality ones. Bad adapters (old,Corroded ones) can degrade the signal.  Cables play a very important part, all pro audio users will get the best cables in the market>>> some who dont even like the quality of these high-end cables from the market make their own cables to suit their satisfaction/Quality standards.

On Board Sound
some on-board sound cards can cause some problem when it comes to unwanted noise.
It can pick up stuff from the CPU fan or the mouse movements etc. So it is highly recomended not to use on-board sound card. (perticularly when you want to record a lot of live stuff ie instruments, vocals etc), even though there are pretty good on-board sound cards(NVidia Chipsets) on Giga-Bite and Asus motherboards. Which give 5.1 Dolby sound on 24 bit processing.

Amplifiers(to be or ..)
It is better to use the external aplifiers specially for Mics and other Acoustic recordings. the external amplifier will give much lesser hiss and likely to sound better with superior frequency response than the internal sound-card amp( in most of the cases of low end sound cards). It is always good to have a nice set of amplifers if you are a serious musician and want to record vocals and other acoustic instruments.
When connecting a mixer (or any other device) to your soundcard, there is a mixer output amp and the soundcard input amp. it is safe to assume the soundcard amp is generally the lowest quality in the studio. So keep the soundcard amp gain to the minimum, and give all the gain through the mixer or the specialized amp for that instrument or the mic.


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