Do you hear what I hear?

I drove my wife's car (Subaru Forester) yesterday at highway speed and there's a slight whine in the engine during acceleration. It's not there during idling or at steady speeds and you wouldn't even notice it if the radio is on.  There's no noticeable drop in power and/or fuel mileage.  The only other clue I have is that there's a slight hesitation during starting.  I checked the battery voltage and it's OK (12.8V).

I typed in "whine during acceleration" into Google and one of the results was "Whine during acceleration (wife not in car)."  Heh.

Some of the auto forums online say it might be the alternator and others say check the power steering fluid.  I'm hoping it's not a leak in the manifold although I think I would hear that during idling.  I'll have to check everything hooked up to a belt.  Anybody else have a similar problem?  Thanks.