Most older homes that have radiators encounter the snapping sounds when the furnace comes on and the vertical pipes begin to heat up. The noise is caused by the pipe rubbing against the floor as it expands. To eliminate this noise be sure the pipe is centered in the hole. If you need to make a minor adjustment on a radiator or two a firm push with the leg is usually all that is needed. If the pipe cannot be moved or returns to its original position, take the cardboard tube from bathroom tissue, split it length wise and place it on the pipe. Slide the tube down the pipe into the hole until it is just below the surface of the floor. Now the pipe slides against the tube instead of the floor and the snapping is gone.