A 3-month-old two-story brick house has a repeating one-at-a-time milisecond sound similar to a large drop of water onto a hard but resonating surface or a wood-on-wood low-pitched tap or creak. It repeats as quickly as two seconds or at an interval as long as 20 seconds and has been fairly constant for about three weeks. It can be heard upstairs and downstairs in the interior walls in and adjacent to the up and down bathrooms. The weather has been in the 30's and 40's. Please help.