[JackMS]

It is the Evaporative Emission system. About 5 hours ( I think) after shutting off your truck, a vacuum pump turns on to pull a vacuum on the system to check it for leaks. There is a rubber hose, about 3/4" or 1", that goes from your fuel fill pipe to a charcoal canister under the truck. All the vapors coming off the fuel in the tank go through the charcoal and are filtered out instead of just going into the atmosphere. I think when the truck is running, the engine provides the vacuum. Been a while since I worked on it so a few of the details could be wrong, but that is basically it as I understand it.