the pair of upper and lower enlarged diameter portions 38a is set so as to be larger than the total wall thickness of the partition 28 and the abutment plate 30 thus allowing the vibrating plate 38 to vibrate vertically by a very small amount a metal plate 40 is secured to the center of the diaphragm 14 the metal plate 40 facing the vibrating plate 38 the metal plate 40 is rigidly secured to a plunger 44 of a cylinder 42 which is in turn rigidly secured to the base plate 12 so that tee metal plate 40 is movable between two positions that is a first position where as shown in fig1 the metal plate 40 is spaced apart from the vibrating plate 38 to allow the latter to vibrate and a second position where as shown in fig2 the metal plate 40 is brought into contact with the vibrating plate 38 by the operation of the cylinder 42 to fix the vibrating plate 38 on the partition 28 in order to prevent the plate 38 from vibrating the cylinder 42 is connected to a hydraulic pressure generating source 60 through a stop valve 62 controlled by a control means microcomputer 66 which is supplied with a signal from an engine frequency sensor 64 vibrations of the engine.