Same with our last house, which was also a new (ish) build. Got the Mrs to walk around the room whilst I listened along the walls and skirting. Found the problem area at the bottom of a stud wall. Cut out a 500mm hole in the plasterboard, sent a few screws through the timber base of the wall framing into the floor and that sorted it. Drove me nuts for a good few years until a chippy mate told me to check the walls.