"Moaning" pipes in new construction?
Ever since we moved back into our house after extensively remodeling it, including almost all new plumbing, we've noticed (especially at night, when other sounds are low), a frequent "moaning" that lasts a second or two at a time. It can be quite intrusive when the house is quiet. We haven't been able to identify the source or a trigger for the sound, but our best guess is that is has something to do with the water pipes. For the record, we have tankless water heaters (two) serving the house and a small apartment. The sound occurs even sometimes when there is no obvious water use going on, but we just don't know what else could be causing it.
We are quite surprised to have such a problem in a house with new plumbing. Is this a common/known problem in new construction? Is there something we can do to fix it, hopefully without tearing holes in the new walls? Thanks for any ideas.