The problem started nearly three years ago when my neighbors had their crawlspace water pipes replaced and their kitchen and bathroom plumbing redone.
Separating them seems too daunting (there's foundation concrete that would have to be broken up) and not necessarily a fix for me anyway.
Since the pipes run under the center of the duplex in the crawlspace, I think I'd hear it loudly even if the pipes weren't touching.
The neighbors took a good first step when they followed their plumber's suggestion and replaced the pressure regulator, but since it didn't do the job we'll toss out some possible causes and suggestions that may bring you peace and quiet.
-- Valves that are not fully opened under a sink, a toilet or at the washing machine can sometimes cause moaning pipes.
Old rubber washers shred and little bits of rubber can get stuck, interrupting the flow of water and causing the moan.
-- It could be that one of the new pipes that was installed is rubbing against wood where is it attached to the floor joists or passes through holes in the walls.
