The problem to be solved by the present invention is to clear up an eccentric load onto a pin and a problem of deformation and cavity due to flowing material during extrusion when spring washer blind-holes in a piston for an automobile transmission are formed by extrusion with a pin. The noise sensing section A includes a time constant circuit to gradually vary a power supply voltage including impulse noise. A lowering frame is fixed to the lifting frame so as to be displaceable beyond its free end and the other part of the bridge section is provided in the vicinity of its free end with outwardly pivotably mounted, length-adjustable trestle legs.