I had the same type of noise you are describing,and it was the belt slightly out of alignment.  It annoyed the Heck out of me.  It only did it, like you said, when excellerating.  The alignment marks on the rear of my 06 S40 are not exactly true.  I had the dealership check it out for me, and that is what they said.  Oh...they checked that and adjusted the belt for free.