Did this just start? Do you have hot water heat? Could it be the heating pipes expanding and contracting and the noise is caused by a tight restriction when the pipe goes through something? Could it be coming from a neighbor? If forced air, do you have a humidifier? It could be turning on and off. What about an ice maker? do you know what the pipes are made of? Copper, cpvc, pex? Do you know what your water pressure is? Sometimes a PRV can make some noises. What about a check valve? Got one? Sometimes they are on the water meter or close by. Do you know if you have one?