I have a 01 3.9L 4x4 dak, and when I first got the truck I threw a CEL P0042?? I think??(Major emissions evap leak anywasy) that turned out to be a split hose by the emissions service port. I replaced the vacuum lines that were anywhere near here as they were all cracked. The trucks been fine ever since, (this was over a year ago) but when Idling, I can hear what sounds to be a vacuum leak still, no code. I've pinpointed it's location to be right beside the battery on the part where these 2 vacuum lines connect to, but am unsure as to what this part is. It has a plug on it, and when I disconnect the plug the sound stops, plug it back in and it makes noise again. Is this normal operation or is it malfunctioning? Sorry for the long post and lack of correct terms, hope somone can help.

 

The thing you are talking about is the purge solenoid. As far as I know you shouldn't be able to hear vacuum from it, but if it was to fail it would throw a code.