Could this be a ghost

So I never believed in ghosts but recently my girlfriend and I moved into a new place. Some strange things have been happening. For instance, we didn't have the bedroom setup so we slept on the couch in the living room for about a week. Around 3 AM my dog and I would be awaken by one bang on the front door. I figured it was some ding dong ditchers but this happened 3 times around the same time and it was just one knock. Then on saturday night I had a couple of friends over and about 3:30-4 in the morning we were on the couch and the front door opened and closed. One of my friend says he was in the bathroom at the time so I'm not sure if he was just messing with me or not. But then last night my girlfriend and I were awaken around 1 am by my dog growling down the hallway, after I got his attention and had him come lay down I went back to sleep. I was awaken once more by a brush across my neck/chest and got the chills at exactly 3:02 am and when I looked over to see my dog, he was in the closet sleeping, kind of like he was hiding or something. Real strange stuff, but what do you think? Maybe I'm just being paranoid...

no, dogs are known for having more loyalty than cowardice. Dogs would attack a bear to defend the rest of the pack (in this case, you and your girlfriend). I doubt a ghost would scare a dog so much that it would hide in the closet. More likely if the dog was scared it would try to wake you up and stay very close to you. The dog sleeping in the closet tells me that the dog was actually not scared at all.
Having your sleep disrupted makes you vulnerable to hypnogogic and hypnopompic hallucinations, so anything that you heard, felt or saw while waking or falling asleep after being disrupted would likely be a hallucincation. These are actually a lot more common than most people know.
The knock on the door was probably due to the temperature changing. The materials in the door will contract some as its cooled in the night. Perhaps the bang is a moment where the contracting is hitting some critical point where the lock or hinges begin to put pressure against the wood. The expansion and contracting of wood can cause all kinds of knocks and bangs around the house. For example, at my house, in the winter when our furnace kicks in at night, the hardwood floorboards start to expand as the heat moves through the hall. As each board expands it sounds like someone is walking down the hall.