I have a vehicle that does the same thing and it was just the idle of the motor making the exhaust slap against a piece of tin that kinda wraps around the muffler close to were the muffler bolts to the motor. Can also u a long rod as a stethoscope and put your ear to one end and touch the other end around the spot where you hear the noise. Do this while the vehicle is idleing. Please be careful and don't put it in a spot were something is turning or moving.