Q: Why do I hear a scratchy sound in my mixing?
With an appropriate tone of seriousness, let us assume that this is a real and genuine problem and consider the possibilities...
In my experience of audio problems going back many years, I can say without a doubt that most are caused by faulty connections. Pull out the plug and push it back in again - that normally does the trick, pending further investigation.
As we transition from analog and non-error-correcting digital connections to computer-style interfacing, connector problems are becoming fortunately fewer and fewer. (Although anyone who ever used SCSI will have quite a few painful memories.)
Even so, it's a rare system that doesn't have analog connections somewhere, so I would recommend checking them first.
Another problem that could be interpreted as a scratchy sound could be interference. I once lived near London Heathrow Airport and every few seconds there would be a 'zzzit' noise through my hi-fi, presumably picked up from the radar installation. I have heard from other people about this problem too. The usual advice is to construct a Faraday cage, although unfortunately I have never known anyone who has actually done this. If you have, please let us know how it worked out.
Analog faders are very prone to scratchy sounds. A fader has a resistive track along which a conductor moves, thus acting as a potentiometer and picking up a voltage corresponding to the physical position of the fader knob.
The track can be made of carbon or conductive plastic. Carbon faders, in my experience, never lasted long before they started to scratch, while conductive plastic faders seemed to last forever.
There is a design issue here - if the electronic circuit designer inadvertently allows direct current to pass through the track, as well as the signal, then scratchiness will inevitably be the result. How do I know that? Because I've done it (but only in a prototype fortunately).
Analog rotary controls can be scratchy too. Since they are exactly like faders but curled up into a circle that isn't surprising. But since they are not so often moved during mixing, scratching noises are not so much of a problem.
Automated mixing systems of days gone by could cause what's known as 'zipper noise'. It is caused by having insufficient different possible levels, so as you move the fader, the signal level 'stair steps' from one level to the next. It sounds like a zip fastener being done up, hence the name.
As for other potential sources of scratchy sounds in the mix, well apart from if you're using samples taken from vinyl records, I don't think I know of any.
So, any more causes of scratchiness? We would love to hear.