Suddenly I have a loud knocking noise in my hot water lines only. 3 faucets upstairs cause the knocking in the subfloor and a wall down stairs. No loss in pressure or temp. only from the 3 hot water faucets; doesn't effect my shower or down stairs bath or kitchen. House built in 2003, wirsbo used with pex lines. I have tried to express any possible trapped air, but it has not changed the problem. I have a 1 y/o water heater and prv. The noise stops a few min after the water is turned off. Is this dangerous? Will a leak be inevitable? How can I fix this? Any ideas?

Explain knocking. Do you mean a loud bang when closing hot water faucet or do you mean a rat-a-tat-tat when running hot water.

If you mean rat-a-tat-tat, close cold water input to water heater about half way and see if that changes anything.

It can be number of things. I would first Closed and Opened all angle stops in your house. Than go after faucets with loose handles. Then look at tub/shower handles... Back to you... Milo

Thank u so much for writing back!! The water lines bang against the floor boards upstairs; I fell the banging in the walls as well. Only the hot water and only on the 3 sinks. They are also not back to back but are separated by a bedroom, laundry room and many walls. I had the water heater replaced over a year ago. It feels like water hammer, but the valves are open and it stops soon after they are shut. I have no loss in pressure. I have turned the water heater down, I have drained the lines and filled them up again. I replaced the prv before the water heater. I'm out of ideas. Could the taps have become faulty? I have replaced the seats and springs on the bathroom sinks. (after they became leaky a year ago). Please help!

I will give the angle stops a try. Any chance all 3 angle stops on the hot water lines have become faulty at the same time? This does not happen when I use my shower or tub or any other water discharge.

Banging like water hammer, but it happens while the water is running. Stops soon after turned off. Sounds like the house is settling,but very loud and frequent! I think it starts in the line that feeds the water heater and then continues from there

Go faucet by faucet. Open water One by One. Determine what faucet is running when banging begins. That's the source of the problem.


No, it is 1 ( one ) angle stop that does it. We don't know which one. Once you move them (close & open) , they will reposition and banging will stop. Make sure you leave them open all the way. Let me know what happened... Back to you.. Milo

Water hammer is one loud bang when a fast acting valve (such as a toilet) shuts off. The sudden stop of water flow causes the pipe to bang against a structural framing member.
A loose washer can cause a rattling noise. This noise will occur when the water is running. The loose washer can be in any valve. If you have a loose washer it's in the hot side.
Changing the position of a valve (closing it slightly) can change the water flow and stop the banging, therefore help you locate it.

Since you said that this occurs when any one of 3 sinks are used it could be the cold water input stop valve on the water heater.

You can also put you hand on the pipe to attempt to feel where the vibration is the greatest.

I will give it a try and get back to you. The inline ball valve that supplies my water heater has a slow drip coming from the handle. Any chance that is related?

Is there a valve that spits the line- main line going up to the second floor and splits to each of the sinks? If there is; could that be the problem?

is there a valve that spits the line- main line going up to the second floor and splits to each of the sinks?

Not likely.

If the stop valve is truly a ball valve, it should not be the problem. Ball valves do not have washers. You can stop the leak around the stem by tightening the bonnet nut slightly. Are you sure that it's a ball type valve and not a compression type.

Another approach to locating the valve would be to close each hot water stop valve, one at a time and running some hot water to see if the knocking stops.

Milo
What do you know about loose Pex connections and how well does Pex transmit sound