One night about a month ago I was watching TV when I heard a loud metal sound followed by the sound of surging water. It was so loud and unsettling my dog started barking. I went next door to ask my neighbor, who had just moved in, if she was running her washer, which is upstairs. Or taking a shower. No answer. I hear water running in the pipes closer to her wall. The HOA management people had their Mr. Fixit man come out. He looked for signs of water close to the dishwasher and under the stairs. He listened to the walls and said he could hear it where I said it was. He could not find any appliance running. Their next step is to have a plumber with a listening device come out to find the leak. The water sound continues. Still not telltale signs of water buildup anywhere. In addition to the constant sound I am not fretting over the possibility I may have damaged the pipe when I tried to put in my molly bolts to hang a small cabinet. Is that possible? The condo was built in 1968.
It's quite possible that some other unit is draining into the same pipe. Typically water sounds are generated only in the drain pipes, because the inlets are systolic (i.e. always at peak pressure). Since at least so far nobody's reported leaks or water damage, my uneducated guess is that some faucet or toilet valve gave out, and there's constant water flow within the system. Whoever lives in the unit where this happened may simply be unaware of (or doesn't give a darn) whatever plumbing fixture has water running into and out of it.