darensml320

My 1999 ml320 is making a odd tapping knocking sound but the odd thing is that you can only hear it from the inside the car i listened to it from under the car and under the hood but cannot hear it but im pretty sure its coming from the engine cause as i rev the engine it goes with the rev but then again it also will go away then come back also when it does this i can cut the engine off and start back up and the noise is gone HELP please