let me guess, you're under the age of 30?
a dell laptop I had had the same problem with the adapter. no one else in my office seemed to be affected, simply coz their hearing had lost being able to hear the high pitch. it drove me nuts.
there's not much you can do about the adapter, although I have read that it can be sometimes due to the power supply in your building, someone figured he only had that problem in a certain room in his house.