2011 Nissan Frontier Engine is louder after startup

Recently I have noticed that my engine seems louder when starting up. Last night I had trouble starting my truck, and had to crank it a couple of times to get it to go. I sounded like it was out of gas, but wasn't. Also, a few weeks ago, when I started my truck, I heard a clicking noise that quickly disappeared. I hadn't driven my truck in a few days at that point. Recently I had to get a new battery, and lately I have noticed that there is a vibration occasionally when accelerating around 45 MPH. This may have always been present, and I bought the truck brand new in 2011. I have also noticed that the rpms when first staring the truck are high, and if I shift through the gears, they increase for a moment if sitting in that gear. Also, this may or may not be related, but when I turn the truck to the "on" position, the battery looks half charged, then when I turn it fully on, the battery gauge moves up a little. There are no fluid leaks, dim headlights, check engine lights, or grinding noises. The weather is starting to change, and the temperature can swing up to 30 degrees within the same day. I am getting my transmission flushed tomorrow, as well as the differential fluid changed. Oh, and one more thing- once the engine is warmed up, the rpms do not rise and fall when shifting gears, and there are no vibrations at idle. I have had this truck for 3 winters and don't recall this much happening when the weather changes over. I know this is a lot to cover, but I wanted to get as much info across as possible. Thanks!