Volume rising and lowering incrementially

Hello all,
This may be a audio hardware issue or perhaps a driver issue and not a vista
issue, but I thought i'd through it out there anyway. When i'm listening to
music with my laptop running vista SP1 32 bit, I can hear minute changes in
the volume. It goes up and down in a very small range, but its noticable. It
happens regardless whether i'm using windows media player, another app like
itunes or streaming. Any ideas or input would be appreciated. thanks!