There's no question that well-played electric guitar adds excitement to a song. Taking the quiet pluck of a guitar string, however, and filling an arena with soaring notes requires many steps before being unleashed by speakers. With guitars, electrical noise comes in two forms: intermittent crackling and steady hum. Both are simple to troubleshoot.
Plug the guitar into the amp with a single guitar cable -- no effects or pedals in the signal path. Guitar cables are the most likely culprit for this kind of noise. Starting at the plug in the amp, wiggle all parts of the cable up to the plug into the guitar until you can cause the crackling to occur under your control. Replace the cable with another and repeat. If there is no noise, the first cable is the problem. Repair or replace the cable.
Check the jack on the guitar with several different cables. If motion on all plugs creates the same crackling, the jack itself will need attention. Check to see that it is mounted tightly. If the noise continues, the jack will need service.
Rotate each knob on the guitar. If one or more knobs cause an intermittent crackle when turned, there is likely a buildup of oxide inside the potentiometer to which the knob is attached. This may be corrected with a contact cleaning spray, or the potentiometer may need to be changed. Refer to a guitar technician if in doubt.
Shake the body of the guitar if the noise remains. Motion from the body indicates loose wiring inside. This may include the fine copper wiring of the pickups.
Move the guitar in relation to the amplifier and note if the hum decreases. Particularly with single-coil pickups, it is normal for a guitar to pick up electrical hum from sources such as transformers. Changing position should minimize such induction.
Remove both hands from the guitar. If the humming grows louder, it may indicate a ground loop. If your amp has a "Ground" switch -- most likely on the back -- flip it and repeat. If there is no switch, try the amplifier in another wall outlet. Do not defeat the ground pin on the amplifier's plug. If the hum continues, this may indicate an amplifier problem.
Replace guitar cables and evaluate hum. A cable that causes more hum may have a broken or loose wire. Repair or replace the cable.
Eliminate other possibilities. If the hum remains, internal wiring may be an issue.
Check Internal Wiring
Open the wiring cavities on your guitar by removing the screws from the appropriate plastic plates, including the pick guard, if necessary. Some guitar designs may require strings to be loosened or removed first.
Visually check for loose wires. Using the wire's length along with evidence of a connection, determine where the wire belongs. Frequently, ground wires are attached to the body of a potentiometer with a touch of solder. If you see such a spot with no wire attached and possibly a few strands of wire, that is likely the contact point.
Check for pinched or broken wires. These will need to be replaced with wire of similar gauge. Soldering provides the most reliable electrical connection. If the fine copper wire of a guitar pickup is damaged, the pickup itself may need to be replaced or re-wrapped. Refer such a problem to a guitar technician.
Guitar circuits are generally simple and can be addressed by someone with basic soldering skills. Some instruments using active electronics contain circuit boards and are serviceable only by those with advanced skills.
While guitars have tiny amounts of current flowing through them, it is possible through wiring problems with an amplifier, particularly older models, for serious current potential to be introduced. Touching another device such as a microphone while touching an affected guitar may cause severe shocks. Loud humming in the guitar-amplifier chain may indicate this, so take care when diagnosing noise problems. Consult an experienced professional whenever in doubt.