Got a refurb unit. When the unit starts up and there is that "Welcome to Mac" chime, the left speaker growls/buzzes/has distortion .....even at 1/3 the volume. When watching a movie, I can hear some distortion too (usually during louder, intense scenes). Is this normal? I think that it isn't... so what is the problem and how can I fix it? I've had notebooks before and this is the only one that his this problem to this degree.