2003 Toyota Camry Grinding like noise during acceleration

I am getting a sporatic grinding noise during acceleration sometimes with the car slowing when it happens. I have had it checked out at the dealership but they can't seem to find the problem because they say they can't reproduce the sound. It is loud when it happens sometimes it happens one every 2 weeks and then sometimes it happens 3 times in a week. I don't know what to do about it. Can you offer some suggestions?

Thanks