There is a loud noice in middle of night when either of the two toiliets have been flushed. This would be the first flush in several hours and does not happen again if another person goes to the bathroom shortly after. The noise is loud & squealing. It's very hard to tell where it's coming from because it sounds like it's in the pipes or traveling through. Possibly even coming from the hot water heater located in the garage directly outside our master bedroom & bath. Please help.