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.

Any chance its your toilet that's leaking into the bowl (flapper valve worn out)... and the water you hear is actually the cold water?

Gas or electric Heater?
I open the closet and hear water running in the pipes Is this water running all the time? Back to you, Tom

CMW said, " live in an apartment and my water heater, ac unit and pipes are all in one closet"...

I'm going to guess that also means the forced air furnace is in this room, too!

If this is the case, then the running water could be the condensate pump sending condensate from the furnace off to a drain pipe (usually at the washer drain). The knocking noises could certainly be sediment build-up at the bottom of the water heater and the knocking sound only shows up when the water is getting heated... maybe?

Check the heater when it is heating and when it is not heating, CMW and see if you can isolate the knocking to heating water heater vs water heater in stand-by mode.

Check for the running water sound when the heat is working (or AC)... see if you hear water flowing during the cycle.

Only other thought is for you to look at the temperature and pressure relief valve (T&P valve) at the water heater... follow the pipe coming from that valve (on top or side of the water heater) and see if water is coming out of that pipe. If water is coming out of that pipe then you need a new T&P valve!

Back to you...

Mark