New fan motor noise on Denali. Can't figure out what it is.

I have a 08 Denali that we bought this past winter. Run fine, mechanically sound w/ no problems. Since it's been warmer when I start it, it sounds like there is a louder fan motor coming on. It's noticeably louder. I shut off the whole heat/ac control thinking it was a fan motor and it was still running. I have no clue what this running motor could be. Anyone have any ideas? It sounds like it runs fine and has a job to do but I can't nail down what it is and its driving me nuts. It's very noticeable when you start the car.