When im driving its making a rattling noise it does it when its idling to just not as loud when i reach 4000 rpms it goes away and my gas milige has gotten worse none of the warning lights are on in the dash and when you do a systems check everything comes back good transmission seems to be shifting fine i thought it was a pully but i took it to get checked out and i was told it was the valves but i dont think it is because the rattle would never go away it would be constant