When my truck warms up to operating temp on the highway I can hear what sounds like a vacuum leak coming from the passenger side of the truck. It's intermediate while driving. No performance issues or anything of that nature. When I get home and kill the truck the "vacuum leak" sound persist for about 2 min or so and finally stops. What could be causing this? I don't have much experience with Cummins so I'm at a lost. Any help would be greatly appreciated.