If the sound is intermittent and seems to come from the plumbing system, it is most likely caused by a leaky toilet fill valve. To find which toilet is leaking, you will need to remove the lid from the tank and adjust the fill valve mechanism until the noise stops. Once you have determined which toilet is leaking, replace or repair the fill valve.
If the sound occurs when you run a faucet at your sink, tub, or shower, you may have a faulty washer. It is a good idea to change the washers in your faucets approximately every five years.