alrite ya'll.91 accord 4dex.i recently change the motor oil for about 1month.my accord runs pretty smooth.when i open the hoodup to listen when the engine runnin at idle i hear this pop pop grain sound coming from the motor?what can it be?is this normal?the car drive fine.is not a knock if it is i would know.the sound is not that loud but pretty noticeable..like something is popin in the motor?