I am also experiencing the same thing as ottoman1384. While the noise is happening, I notice that changing the speaker volume has no effect on the loudness. It also only happen during the first few minutes of powering up.
I remember reading a post in one of the sub-forum about noise coming from the screen. It suppose to be caused by poor insulation / interference between the screen and the cpu. Lately, I have been wondering if that is the cause of what I am hearing. My current install is with 5.2 img.