THIS NOISE TAKES ITS SPELLS .ONLY DOES IT AT IDLE .FIRST I THOUGHT IT WAS A VACUUM PUMP AND PULLED THE BREATHER LOOSE AND THE NOISE IS COMING FROM AIR INTAKE HOSE.CRACK THE THROTTLE AND NOISE GOES AWAY AND MAY NOT COME BACK TILL A FEW HOURS LATER AT IDLE.FOUND BOTH EXHAUST Y PIPES IN NEED OF REPLACING LEAKING BETWEEN GASKET AND PIPES.ALSO FOUND NO.5 INJ NOT FIRING TILL REACHING OPERATING TEMP.ONCE THE NOISE STARTS IT IS A STEADY THUMPING NOISE.