If your kitchen faucet makes an unusual hissing noise, there are a number of possible causes. To identify the most plausible explanation for your broken tap, pay close attention to the exact quality of the hissing sound. Be alert for any other unusual signs that accompany the sound, such as a rattling, vibrating or change in the stream of water from the spigot.
A Worn Washer
The problem with your faucet may have virtually nothing to do with the plumbing system itself and may be due to nothing more serious than a washer that is poorly attached or screwed on too tight. Listen for the exact quality of the hissing sound; if it's accompanied by a slight vibration, you probably have a worn washer. To replace it, first turn off the water from the source and remove the faucet by the stem by taking off the retaining nut and taking out the washer beneath.
If changing your faucet's washer doesn't relieve the sound, your hissing tap may be indicating a water pressure level that's either too high or too low. If this is the problem, you will also see evidence of the pressure level in the manner that water runs out of the spigot. Compare current pressure with that of the past. If you suspect that the water coming from the supply line needs to be readjusted, contact a fully-licensed plumber for an inspection.
Water Filters and Attachments
The problem affecting your kitchen faucet may be even more superficial than a washer within the faucet assembly. If you extended the faucet in any way, by adding a faucet-mounted water filter or a spray attachment, a poor connection may produce a hissing noise when the water is flowing. With the water on, pass your fingertips 360 degrees around the water filter or spray head. If you feel any fine spray of water or jet of air escaping through a crack in the housing, it's likely the cause of the hissing noise.
Other Faucet Noises
Another noisy problem that can affect kitchen faucets is called "water hammer" because of its loud, percussive quality. If you hear this noise while the faucet is shut off, there is probably a pressure-related problem within the tubes of your plumbing. Generally this occurs when the water supply has been suddenly cut off. One solution to water hammer is to install an air chamber or a water hammer arrester within the plumbing, which serves to relieve built-up pressure, quieting the knocking sound.