I have a '74 280C that has gear whine at around 60 mph also. Like yours, it disappears if you go faster than about 65. It's been whining since we got the car about 5 yrs ago, and hasn't changed at all. We have put several freeway trips on the car of over 500 miles each, and the whine stays the same. I suspect that the bearings are worn in the differential and could stand to be replaced, but I did purchase a spare differential from the local used parts emporium just in case - it only cost $35, and is a great worry easer. (It's still on the shelf in the shop...) One thing to note is that there are a lot of M-B differentials that all look pretty much the same but have radically different gear ratios, depending on the engine and car, so try to get one with the same ratio, so your speedometer and odometer will be accurate. The car is noisy at 60, but my wife drives it the most, and she rarely goes over 55, so we haven't had a problem! My '82 300TD with the 4.3L V6 installed goes at any speed from zero to well over the top peg on the speedo with no noise at all - the differential in it looks the same, but has many more miles on it. Possibly the turbo diesels are easier on the gears and bearings due to the lack of torque at low speeds, compared to the gas engines? It's an interesting question...