Guitar amplifier hiss is almost always the result of external applications. Amplifiers are almost silent when nothing is plugged into them. The more devices and cords plugged into an amp, the more likely it is to make noise. Other causes of hiss are grounding problems, external effects units, bad cord ends or over-driven gain controls. Use a processes of elimination to locate the source of the problem.
Plug in your guitar and all effect units that you normally use. Turn the volume, gain and all processor controls to your normal settings. Start at the amplifier and begin pushing sideways gently on the shiny ends of the cables one-by-one. Push the guitar's cable first, and then work your way through the patch cables until you have tested all of them. When you push on the cable end, you will either hear the hiss subside, or there will be no change. If the hiss subsides when you push on the cable, the cable is bad, replace it.
Unplug your effect units. Bypass them one-by-one and plug your guitar into your amp without them. Listen for the hiss to subside whenever you eliminate an effect unit. If the effect unit is bad, is causing the hiss, replace it.
Check your guitar's external power cords. If you are running a long extension cord, take the extension cord off and plug the amp directly into a wall socket. If the hiss is gone, your extension cord is causing the hiss, replace it. If your amp is still making a hiss even when you plug it into a wall socket. Carry the amp across the room and plug it into a different wall socket. If the hiss subsides, the wall socket is causing the hiss. Wall socket and external cord hiss is caused by grounding problems.
Turn your amp on after you have checked the cords, effects and wall sockets. If your amp is still hissing, check your controls. Your volume control should always be set lower than your gain control. If your volume control is set higher than your gain, you are over-driving the amp. Turn your volume control down lower than your gain and your hiss should dissipate.
Check your guitar. If you have eliminated all cords, effect units, amp settings and continue to hear a hiss, turn your guitar master volume up and down. If the hiss subsides when you turn your guitar master volume down. Your guitar has a grounding problem. Take it to a guitar shop where a technician can locate the bad ground and fix it.
Wah-wah pedals are common causes of amplifier hiss; if you have one in plugged into your amp, check it first. All guitars with single coil pickups such as Fenders are inherently noisy. You cannot fix it. If it bothers you, switch to humbucking pickups such as the ones used on Gibson guitars.
Don't touch other devices like microphones while you are holding your guitar if you suspect that your system has a grounding problem. You could sustain an electrical shock.