Got a 97 150 w/ a 4.6 automatic. Since I've had it I've heard a squeeking sound when it shifts from drive to overdrive. It's only a 1/2 to 1 second noise but it has started to do it at random going down the road in all gears at any speeds. Local mechanic couldn't find the source of the noise. Looked through service manual and ran across something having to do with a 4 wheel drive vacuum shift or something related to that. Any help is greatly appreciated.