When your band plays in different locations on a regular basis, noise in your P.A. system is a very common, and sometimes a very difficult problem to solve. Noise comes from many sources. Some of the most common ones are:
1. Poor cables on the inputs
2. Fluorescent lights
3. Dimmers
4. Poor design, particularly shielding and location of the mixer and power amps power transformers
5. Poor A/C wiring in the location where the P.A. is being used
6. Radio stations or other transmitters in the area
7. Large motors close to the system
8. Grounding problems, when using equipment manufactured by different companies
9. Noise in the input signal, especially guitar pickups
10. Magnetic fields induced by other nearby components like air conditioners
Be sure to turn down your level sliders before you attempt to plug or unplug any source that is connected to your system, or you may produce damaging pops or clicks that can take out speakers.
Most of these system problems can be reduced or even eliminated by using high quality cables and balanced lines.
To check for the source of the noise, unplug all inputs at...