I have a '04 BA 4 speed auto NA 6 flat tray with 30,000 fm on the clock. Lately I have noticed a vibration goes through the car when slowly accelerating in 4th gear. Do these gear boxes have some sort of torque converted lock that kicks in under low load in 4th gear as there seems to be 2 different rev levels when doing approx 100k in top gear. The vibration starts when the revs are at the lower level. Any thoughts?