I just bought an 06 Legacy 2.5i in february, and i have just recently noticed an increase in noise coming from the alternator. This is probably due to the sound system I have put in the car. I notice when I turn the ignition switch just to have the battery on, I hear this consistent whining noise. When I turn the car on, the alternator begins to whine. This may be linked to a bad battery? This is such a coincidence because I recently recieved a coupon for testing/charging my battery at my local Subaru dearlership! 
I have not noticed any issues with the lighting/slow ignition/decreased performance of the sound system yet. I am definetly buying a voltmeter tomorrow to check the voltage.