QuestionQUESTION: Most people complain about a clunking noise from the rear end. I have that too and have replaced the thrust washers and the drive shaft u-joints. No change. It still clunks but what bothers me most is a metallic knocking or rattling on deceleration at speeds above 40 mph in top gear. The bore on one of the pinion gears is a little worn inside, the other is tight. With power on, power off there is this rattle or knocking noise. Is it this gear or something else. I know these gears are no longer available. 1978 MGB
ANSWER: Hi Allen,
If you mean the spider gears on the short shaft in the differential, they are not moving when going straight on the road. The only time they rotate is in a turn.
You should try to determine at what speed the noise is to try to locate it. Wheel speed? Faster than wheel speed? Any difference in "S" turns on the road? Or in turns? Brakes on? Hand brake on? Any gear? A load in the trunk or not? The noise random or or steady?
It is not a clicking noise is it?
Howard
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QUESTION: The noise is more a knocking noise rather than clicking. It is not noticable in any other gear but 4th above 40 mph. No noise on turns or braking. It sounds like it is coming from the rear of the car. Could it be excessive back lash between the pinion and crown gear. What is the recommended backlash, .008" to .010". It only appears on deceleration just at the point of no load to load. It doesn't happen all the time, and no noise under load. Could it be masking a bad rod bearing and transmitting the noise to the rear of the car. Sorry I am grasping at straws here. Thanks.
ANSWER: Allen, at this point I have no clue what your noise is but I can tell you a few facts. Backlash in the ring and pinion gears would only cause a wine and maybe a random noise at a no load condition and it would not know if you were in 1st or 4th gear. Back lash in almost all ring and pinion gears is about .005"
The engine can not make a noise that sounds like it is in the rear of the car. But because this is a unit body a transmission noise could sound like it is in the rear. On decel the front of the differential is pitched downward changing the driveshaft alignment, But it is in every gear. In the differential the ring gear is now driving the pinion gear and all loads are reversed on the "U" joints. (however, this is true in all gears) When in 4th gear there is no load on any gear in the transmission because it is direct drive. But all gears are spinning all the time except reverse idler. The spider gears in the differential are under reverse load on decel just the same in all gears but they are not rotating unless you turn.
Is this car wire wheels? If so remove both and clean the splines and grease them and replace the wheels and tighten the wheel while off the ground. If it is disk wheels remove the wheels and check the torque on the axle nuts (150 ft lbs)
At first you said it was on decel and now you say it is just between decel and accel? This is common on loose axle nuts on disk wheel cars and loose and dry splines on wire wheel cars. Also, did you check all the large "U" bolts securing the axle housing to the springs?
Howard
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QUESTION: I agree with everthing you have said, however this knocking is not just a single knock on deceleration, it is a continuous knocking at the speed of the wheels until I apply power. This car has disk wheels and I have torqued the axle bolts to 150 ft lbs when I changed the thrust washers in the differential. I also checked the axle bearings when I had the shafts out and they are fine. I haven't checked the large U bolts holding the housing to the springs, but I don't think this would make a continuous noise, but I will check. When I go over a bumpy road I don't here it so I don't suspect loose shocks or springs. Thanks for your comments. Any other ideas are welcome.
AnswerAllen, I have worked in several MG dealerships and never had a car that made a noise that could not be found. You might remove the battery cover to see if while driving you or a passenger might get a better idea where the noise is coming from (right or left or differential).
To find the source of the noise you need to isolate the noise to a closer area than the back of the car, as the symptoms are up to now useless. If it does not do it in 3rd gear at 40 up but does in 4th at 40 up that would rule out the whole rear end as the rear end don't know what gear you are in and drive shaft speed at 40 up is exactly the same no matter what gear you are in.
Howard
