2000 Legacy GT Automatic. Today I noticed a whirring sound from the engine compartment but couldn't localize the sound. I think I guessed correctly though. Tonight when I stopped for a minute, the car wouldn't restart. Got a jump and started but the lights were considerably dimmer than normal and no amount of revving the engine helped. The serpentine belt is intact.

Alternator? Probably, but why didn't I get an idiot light??

The belt could be slipping. But its probably the alternator. A quick trip to Autozone or someplace...they'll test it for you if you don't have a volt meter. A used one...subaru origional...might be better than a cheap aftermarket. Sometimes they tank prematurely. Also check to see if your connections to alternator and battery are tight...but DO NOT over tighten the tiny 10mm nut at top of alternator. Will break the stud. Not good.