Hi and thanks for your response. I took my car in toady to a Mercedes specialist, the diagnosis is this: The bang is caused by the drive shaft mount or bracket being worn, both front and rear flex disk on the drive shaft are bad. When the transmission changes gears this causes the drive shaft to hit the bottom of the car causing the loud banging noise. As far as the transmission is concerned, none of the things that cause the gears to shift are burned out but he suspects that either the vacuum is not functioning or the modulator is not functioning properly. He will try to fix it hopefully without having to rebuild it. So we shall see. Thanks again!