MG Car Repair: Gear Noise, thrust washers, axle gears

Question
QUESTION: I have a 78 MGB that at speeds above 60 kph, when I let up on the gas there is a loud rattling noise that appears to be coming from the rear of the car. Only on deceleration. I have changed out the universal joints, no change. I put the vehicle on jack stands and ran the car to listen for the noise at the diff. and at the wheels and transmission, but nothing. Where else would it be coming from.

ANSWER: Hi Allen,
I can tell you a few things to do to try to pinpoint the noise but not being able to hear the noise I have no way to tell you what it is.
First check for any up/down free play in the differential flange or any in out movement at all. Next try it in 3rd gear and if it is there too is it at the same road speed. Next check the amount of rotational free play in the differential flange. Then jack the rear end up and hold one wheel and see how much free play in rotation the other wheel has. Let me know all of these. Also with light touch on the shift lever do you feel the vibration in the shift lever?
Howard

---------- FOLLOW-UP ----------

QUESTION: There is no free play up,down,in or out on the differential flange. Just a little up,down in the drive shaft yoke up front. With both wheels on the ground the rotational free play on the diff. flange is about 10 degrees. With the transmission in gear and one wheel off the ground the rotational free play on the left side is about 5 degrees and the right about 5-7 degrees. I think there is too much free play. I did the run test again and I think most of the noise is from the differential. I understand there are some thrust washers on either side of the axle gears. Do these need to be replaced. Or is there more to be done. Thanks in advance.

Answer
Hi Allen,
That free play does not seem excessive, but having it different on the right side from the left side is a possible problem. However there should be no up/down free play up front. If the noise is in the differential there are thrust washers in the center chunk that control the free play of the spider gears. You need to disassemble the differential to rebuild the unit and you should purchase a service manual to do this as it is too much to cover in e-mail. If you decide to go into the differential and run into something that is not covered in the manual, I would be glad to help.

With just the rear cover off you can examine it for any problem with the thrust washers in the spider gears. I always removed the differential to rebuild one but you can just remove the shock links and rear shackles and exhaust and just drop the differential down enough to remove the rear cover so you can examine the spider gears for loose play. It would be best to get a mechanic who has rebuilt differentials to look at it as I can't tell you how to decide what is good/bad. It is designed just like all differentials so you don't need a MG mechanic to look at it. Domestic car differentials are designed the same.
Howard