I've got an '06 4.0L with automatic transmission that developed this noise just last week. The truck has 131K on the odo. The noise is audible at idle and when you are driving but goes away at speed. The noise is probably still there at speed but with road noise get's drowned out. I'm thinking vacuum leak but didn't get any help finding a problem with carb spray. I don't see any obvious problems with the vacuum hoses but then I guess some are buried in there. The truck runs fine and there are no codes. Any thoughts will be appreciated. Thanks.