I just crawled underneath the car and looked at the transmission mount. Although it is rather old, it seemed that it was intact and no too bad. (I will replace soon, doesn't seem to difficult). I did notice something unusual, the rubber sleeve next to the the sleeve nut was out of shape. It was pulled away from the nut and bunched up on one side with wrinkles. I am thinking that this would cause the differential not to be perfectly concentric and may cause a hum? I did not feel any vibration though. What do you think?