I think I know exactly what you are talking about. Watch this video at the :30 second mark when I first start up and you will hear it:
https://www.youtube.com/watch?v=lqWmalllpWs
Mine has done that every once in awhile on start up since I bought it. This was the first start up after it was inspected and worked on by Jack's and it still does it occasionally to this day.
I just assumed it was expected behavior but if it isn't... I'd like some more info too.