Hearing noise from hood of car

My Toyota Camry has 148,000 miles and I take excellent care of it. For the last 10,000 miles I've been hearing a rhythmic noise from the hood of the car when idle (at a traffic light) or even when driving. At traffic lights or when idle it's quite loud and I went to the mechanic today - who said it's the bearings from the alternators. I saw the belts which are next to the alternator and they look perfectly fine. Again, the noise was there before, it's just gotten louder in the last 1,000 miles.