M1330 humming when lowering the screen brightness

My M1330 makes a humming noise from somewhere near the left speaker whenever the screen brightness is lowered below 100%.  The lower it is, the louder the noise is.  I think I once came across a forum about this on the internet, but could not find it anymore.  Does anyone know the cause of this problem?  And is there anything I can do to fix this?  This is different from the common CPU whining problem in this model.

Thanks.