Banging, thumping noise in wall. Same wall that the water heater and the water main valve is on. We have turned off water and drained lines. That didn't stop the noise. We drained and flushed the water heater. That didn't work. The noise comes on in the morning and by 1:00 is stops. The noise is constant and does not stop when the water is on. I had the city check the water flow, the pressure, and check to see if there was a leak in our main pipe. Nothing. What could it be? Two plumbers both suggested a expansion tank, but why would we need one now?

How old is your house? What's the house PSI? Describe the noise. Loud banging? Crackling? Your plumbers wan to install a expansion tank as a whole house air chamber. However if your house was built back 40 or50 years ago you might already have air chambers installed and the simply need to be recharged.
One of the "Water hammer" happens when you abruptly shut off the water. You have at least 40 pounds of pressure pushing the water flow. The weight of the water hitting a 90 degree bend causes the sound that you hear. We put in air chambers to cushion the hammer much like the shocks on your car.
Back in the 60's we were required to to put air chambers on all bath lavatories and kitchen sinks. Over time water and condensate built up in them and they must be recharged. First turn off the house at the main house shut off valve. Then open hot and cold faucets at the farthest bathroom to prevent air lock.(This is important.) You will find under each fixture little brass or chrome valves. These are called angle stops. With a small pail held under the angle stop to catch the run off look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut. Put your wrench on it and back it off counterclockwise. Now open the stop and remove the stem and washer assembly, The stop will began to drain. When it has quit draining reassemble the stop and move on to the next one. Do this on each one of your angle stops. There will be six of them in a two bath home. When you are done, close off the faucets you have opened, turn on the water to the house, and give yourself a pat on the back.
You have just recharged your air chambers and saved yourself a hefty service call bill. Hope this helps you out and thank you for rating my reply. TOM

Thanks for your answer. My house was built in 1989. Does your response still apply? Also the noise is pretty constant, not just when we turn the water on or off. Thanks

Does your response still apply?Yes they do. You should have air chambers on every lavatory and on the kitchen sink. Simply drain them and your banging will go bye-bye, Good luck, Tom

How old is your house? What's the house PSI? Describe the noise. Loud banging? Crackling? Your plumbers wan to install a expansion tank as a whole house air chamber. However if your house was built back 40 or50 years ago you might already have air chambers installed and the simply need to be recharged.
One of the "Water hammer" happens when you abruptly shut off the water. You have at least 40 pounds of pressure pushing the water flow. The weight of the water hitting a 90 degree bend causes the sound that you hear. We put in air chambers to cushion the hammer much like the shocks on your car.
Back in the 60's we were required to to put air chambers on all bath lavatories and kitchen sinks. Over time water and condensate built up in them and they must be recharged. First turn off the house at the main house shut off valve. Then open hot and cold faucets at the farthest bathroom to prevent air lock.(This is important.) You will find under each fixture little brass or chrome valves. These are called angle stops. With a small pail held under the angle stop to catch the run off look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut. Put your wrench on it and back it off counterclockwise. Now open the stop and remove the stem and washer assembly, The stop will began to drain. When it has quit draining reassemble the stop and move on to the next one. Do this on each one of your angle stops. There will be six of them in a two bath home. When you are done, close off the faucets you have opened, turn on the water to the house, and give yourself a pat on the back.
You have just recharged your air chambers and saved yourself a hefty service call bill. Hope this helps you out and thank you for rating my reply. TOM

Sorry but no this did not solve the problem. I had stated that we already drained the pipes. But I thought I would give it another shot per your directions. I drained all 17 angle stops in my house. The noise is now a constant hummer with a bang every once in awhile. Can it be a loose pipe?

As silly as this might sound (and doesn't exactly follow your description) have you tested your toilets to see if they are leaking water? Here, you would drip say 20 drops of a dark food coloring into each toilet TANK in the home and then you would wait and see if water eventually shows up in the toilet BOWL. If colored water shows up in the toilet bowl then you have a bad flapper that is allowing water to escape into the toilet bowl and that can activate the toilet fill valve and cause issues very similar to your description.

Try this and then pop back with your results, OK?

Mark

Thanks for all the responses. We finally solved the mystery. A tree root was pressing in on water pipes causing restriction on the flow. It shook everything including the water meter. The city came out and cleared the roots away from the pipes and no more noise. Luckily the problem was on the City's side of the meter so there was no cost to us.

Thanks for the update. Glad you found the trouble a gal all fixed up. Good luck, Tom