I drove my 97 F-250 with 302K to work yesterday because it had not been out since before Christmas. Got home around 5 p.m. temp around 30 degrees. I decided to go ahead and hook up my dump trailer since I was planning on leaving out around 8:30 this morning to get some firewood. It was getting darker so my headlights were on. Backed the truck and trailer up next to my garage and shut it down for the night. I came out around 10:30 to plug up the truck just because the batteries are getting weak (Autocraft golds over 5 years old,PO installed them. I am going to replace with Motorcrafts soon. Glow plugs and relay are new.) Temp was around 25 and it was going to spit snow last night so I wanted instant defrost in the morning. As I was plugging it up I heard a hissing noise coming from the right side front. My first thought was *What the Hxx is wrong now" Looked around and thought is was coming from under the hood, not the tire. Popped the hood thinking what is wrong with the coolant tank, or radiator. Nothing there. Then I thought battery, but nothing there. Then I thought A/C system since defrost was on, northing there. I stood there scratching my head thinking my truck was possessed. I then put my ear to the headlight housing and found the hiss. I reached in front of the battery and pulled out the bulb and it had a little bit of pressure on it. Never had that happen before. But makes sense. Hot then cold. I guess there is a vent in the housing that has probably gotten gummed up or got frozen closed last night. I was so happy that I did not have a tire, radiator, or A/C issue.