Could be your valves. but the common noise is going to be the fuel injectors. thats why you only hear it at idle and low rpm. if it were anything else it would get louder as you rev it. my suggestion as i posted in another form is to pop the hood of ur car fuel injected and put ur head close and listen. i know on my 91 mustang i can hear the injectors loud and clear and its the same noise i get from my bike.