There are quite a few threads on this site about a grinding noise coming from the transmission or elsewhere in the drive train. Just so you know we are not alone, I was riding in a friend's Subaru a few days ago and hers made a real loud howl at any speed from 20MPH and up -- loud enough to interfere with conversation in the back seat. It sounded to me like a rear axle bearing. I suggested she take it to a shop and have it looked at. She said she'd done that long ago and her dealer said it was normal. Sound familiar?