I have had this problem also, I can hear mine when the car is in idle, ie startup. I don't konw if I can hear it anywhere else because I haven't gotten out at a stop sign to listen :-p. I have also put in the "high" octance and it made not a difference. I had asked my dad about it, because I thought maybe it was a spark plug that was not firing completely. The car accelerates with no problem though. I have poor gas mileage as well. I"m thinking it may be the EGR vavle itself, but no engine light.
