'98 with 116K miles. Hearing a twittering/chirping noise when driving. With the truck parked you can hear the same noise. Removing the hose from the top of the valve cover it becomes louder. If you partially block the hole in the valve cover the noise will subside. The noise is definally originating under the valve cover. Thought it might be a pcv valve but could not find one if it has one. This hole is connected to the throttle body I think and has vacum at the valve cover. Any ideas or assistance will be appreciated. I'd rather not have to take it to the dealer unless I have to. Thanks in advance.