In reality they are potentiometers, but they do add to the percieved volume at the end of the chain, which is then sent to the big momma waiting to be tickled (the amp). In simple terms, think of them as bath tubs, each which must be filled to above the "noise floor" (an area at the bottom edge of that particular gain stage) and not too full (above peak, where distorion occurs). By playing with the faders or knobs, you should be able to detect which stage is giving you the noise. If you detect that the offending stage is in a processer in the chain, see if you can remove the noise by dropping the ground out (putting a two prong adapter on the three prong plug) or if the box has a "ground lift" button).

Finally, if that doesn't work, you might want to see about getting a "power conditioner" (a form of power strip that usually mounts over a processor rack with pull out lights etc. - good cheap one come in around $60.). They can be used in place of power strips for sound equipment, and clean up the power, if all your gear is first plugged into them. Properly used, they can clean up about 90% of your noise problems. Good luck!