I realize this is an old thread but I just had to share… my mother-in-law’s plumbing used to moan loudly. It wouild resonate throughout the house as if coming from every splinter of every floor, wall and ceiling. And did I mention it was loud?! It did this for some time and was checked by several plumbers (amateur & pro) who adjusted this or replaced that but to no avail. The family just lived with it. Late night toilet flushes would wake the dead. But is wasn’t just toilet flushes… any water use could cause the moaning.
After much internet research and prior to discovering this site I learned that a faulty or old toilet valve may cause this to happen. I ran water while observing float valve assemblies inside the tanks of all toilets. I discovered that one of the valves bubbled slightly when the moaning occured. I replaced that entire float valve assembly and suddenly, years of moaning stopped.
Hope this can help someone.