There are several types of noise having different effect and causes. I have roughly classified them as follows:

 

1. HUM = a constant low frequency humming sound corresponding with the AC line voltage frequency of 50 or 60 Hz.

 

Main possible causes of hum are:

- not- or bad stabilized power-supply

- (current) overloaded power-supply

- inductor (single coil pickup or wah) picking up an AC magnetic field

- bad ground connection or shielding  

- nearby electroluminescent lamps (Neon & TL)

 

2. HISS = higher frequency constant noise, a sizzling sound. This is a random spectrum of all frequency; called white noise and is comparable to sound of sea waves.

 

Main possible causes of hiss are:

- bad transistors

- high frequency ripple of switching power supplies

- bad BBD chips (chorus & delay effects)

 

3 .CRACKLE = irregular medium frequency noise. It sounds like a fire place burning dry wood.

 

Main possible causes of crackle are:

- bad transistor, especially germanium types have this problem

- bad signal cable

 

4. Radio Frequency Interference (RFI) = receiving radio audio signals, mostly AM band on your guitar rig.

 

Main possible causes of RFI are:

- bad signal cable

- guitar pickups

- gain setting too high

 

5. SQEEL = a resonance of one specific frequency that can increase in volume and or frequency rapidly when not in control.

 

Main possible causes of Squeel are:

- not wax potted guitar pickups

- high gain setting of your guitar rig

- high treble setting of your guitar rig

 

6. MICROPHONIC'S = knocking sound when knocked on or moved. Can also result in feedback

 

Main possible causes of microphonic's are:

- not wax potted pickups in combination with high amp setting

- coil in Wah inductor

- bad guitar cable

- bad vacuum tube(s) in your amp