noise at high speed

We purchased a 2004 Chevy Venture and at 2000 miles it developed a
noise which sounds like a bad beiring. It's an intermittent noise that happens usually over 60 miles an hour, but never when it rains.We have taken it to the dealership on three seperate occations. They found a T.S.B.(trouble shooting bulletin) that talked about a motor mount vibration and installed a damper, with no results. The dealer has no idea what the problem is.