I live in an apartment and my water heater, ac unit and pipes are all in one closet. When I passed by the closet this morning, I noticed a knocking kind of noise. I open the closet and hear water running in the pipes and the water heater making this knocking noise every few seconds. No water is running in the house, like shower, faucet, tiolet or dishwahser, so I'm not sure why I hear water running in the pipes. I looked around and did not see any signs of leaks. The apartment building office is closed for the easter holiday so I can't call them to check it out. I was just wondering if any one had any suggestions as to what the problem could be.