Hi Everyone! Got a bit of a puzzle if anyone could shed some light on the topic.

Florida home. Built in 1986. Wood frame on slab. Two story. Copper pipe. Main shutoff outside the home. Water meter at the road.

Six months ago we started hearing knocking when the water was turned on/off in the upstairs bathroom. Hot and cold water equally. Fit all the parameters of Water Hammer. Gradually the sound was heard more often and eventually was heard whenever the water was turned on or off in any part of the house.

Fast forward to today. Now the sound is constant. It sounds like a constant knocking sound, very rhythmic and never stops. 24 hours with no change. Turning water on or off changes the cadence for a few seconds then back to rhythmic knocking.

What we have done.....

--Turned the main water valve off to the home - sound stops immediately.
--Turned the main water valve on to the house - starts back up.
--Turned the cold water supply line off to the water heater - noise stops.
(Side note... when there is no pressure on the hot water line, the noise is gone. We found that the valve on the supply line to the hot water tank does not seal properly and lets water through even when shut off. Since this is so, when the water pressure rises in the lines due to this defect, the noise begins again until a faucet is open and the pressure is relieved. One more thing to fix!)
--Turned all water off in the house and checked the water meter at the street. Meter shows no usage.
--Followed the recommended procedure for draining of all water from the house (including hot water heater). Noise present as soon as everything pressures back up.
--Turned circuit breaker off on hot water tank and let it cool - still knocking when lines under pressure.

I opened the drywall in a closet adjacent to the downstairs bathroom in question. This allowed me to look into the space between the ceiling of the downstairs bathroom and the floor of the upstairs bathroom (area of the knock). I am able to visualize the copper pipes. Have to say, I was kind of impressed how they had foam around them wherever contact was made with anything such as studs, joists, other pipes. I pulled and pushed and I was not able to find anything that was loose or movable. I could feel the vibration from the tapping in both cold and hot lines.

Any suggestions on how I could proceed? Any other procedures I should try such as the draining of lines? Any info would be greatly appreciated!

Sounds like it could be a loose pipe in the Hot water system....just a guess.

Is there a pressure regulator installed? Has the water pressure been checked?

Often, sediments/residue inside the tank heats up and creates a popping or banging sound. Try draining the tank and see if you notice any sediment flowing out from the bottom of the tank.

Good Luck.

Check water pressure going into house you may need to install a pressure reducing valve (PRV)