In home vacuum system. Low suction. Told leak somewhere behind a wall. How to locate where in what wall?

Put a stethoscope to the wall and listen for air moving. Start at the outlet furthest from the unit, and work your way towards the main unit. As you do this, try to visualize how plastic pipes would be run inside the walls. Keep in mind that the pipes are going to be about 1-2" in diameter, rigid, and will bend at factory bends much like plumbing pipes.