We have this fond suspicion that you should have called a clairvoyant rather than a plumber. The fact that you are in a duplex complicates matters. It is very likely that the noise that you are hearing is coming from plumbing pipes that are in the wall common with your dwelling or even from a pipe that traverses your residence servicing the dwelling next door.

One of the best ways to find a plumbing leak in your fresh water system is by turning off all of the faucets and fixtures in the house, ceasing to use any water. Go out to the water meter that services your home and see if it is registering any usage, (you may need to wait about an hour). If it does, you have a leak somewhere between your meter and the fixtures in your home. If this is the case then we suggest that you call the plumber back and ask him to make a more thorough inspection of the water supply to see if he can locate the leak. If you don't find any problem with the plumbing in your home then this exercise should be performed at the residence common with yours.

Another way to locate running water is by using a long narrow screwdriver as a listening device. Place the metal side of the screwdriver against the pipes in several locations of your home and the handle against your ear. If you hear running water and none of the fixtures or faucets are in use then you have isolated where your problem exists and your plumber can effect a proper repair.