I assume you're cranking the starter when that sound occurs.
Does it feel like the engine is turning over?
If it doesn't feel like it's turning over, it could be the starter solenoid not pushing the pinion gear out. Or the starter motor is going. The closest thing I've seen on subies is on the GC's, is when the starter is going and the pinion gear doesn't pull out fast enough <that's what she said>.
Crazy seeing this on an 08 though.