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MG MGB Technical - Noise from gearbox
|A nagging problem since I bought the car in June 2010. A whirring sound coming from front of gearbox (roughly just under where the radio sits)which increases/decreases with speed of car. The noise is also there when engine is turned off and coasting in neutral. When driving along,If I hold the gear lever when in gear and move it gently in the gate, say in 4th, the noise changes slightly but is still there.|
Any suggestions on what problem could be?
|Is yours an early car fitted with a 3 synchro gear box? If so then this is normal, mine has always done it. It was worse with a wooden aftermarket gear knob, so I put the OE one back on.|
The later cars don't do this.
|4 synchro non overdrive.|
|It sounds like it could be the third motion shaft bearing, assuming that when you were coasting in neutral, you took your foot off the clutch?|
The fact that there is a change in the whining sound when you move the shift lever while driving is significant. You shifter forks in the transmission may be misaligned with the synchro hub, causing the baulk rings to be partially engaged.
Note that if the gear selectors are adjusted when the gear change lever (gear shift lever) is not in its neutral position, then you will not attain the correct amount of travel of the synchronizer hub, which will result in the transmission popping out of gear. Insert the springs, the detent plungers, and the plugs with their fiber washers under their plugs into the selectors. Tighten each of the lock bolts of the selectors (saddles) into their holes in the selector shaft, and then tighten the locking nut, and then safety wire them. If a lock bolt is insufficiently tightened, then the selector (saddle) will vibrate loose, hence the need for safety wire. With the transmission in neutral, wiggle the synchronizer hubs in order to verify that the selector forks (saddles) are properly centered on their synchronizer hubs and are not putting the synchronizer hubs under tension. Next, tighten the lock bolts and safety wire them. Next, replace the three selector fork rod plungers, their springs, caps (fiber washer under the cap head) and their retaining bolts.
|Sounds more like the 1st motion spigot bearing onto the mainshaft cos when the motor is off and in neutral the 1st motion shaft is not moving. The same noise would be heard with the motor running in neutral as the 1st motion shaft is mkoving but the mainshaft is not.|
|J G Bagnall|
|The change in whining when moving the gear lever is very slight but it's there. Re spigot I replaced it when I changed the clutch last year. The noise has existed before, since I bought the car actually. It has not got worse but it has not got better either. I've thought about how to describe in words. Imagine a metal disc that if 100& true there would be no noise - if it was warped at one spot it would rub against something at every revolution, the sound getting faster and faster as revolutions increase and then slower and slower as decrease in the speed of revolutions - all together whether the engine is turned off or on but the car in motion. That is what it sounds like. I've checked the brake discs and hubs, the prop shaft, even the fan belt and the speedo cable connection at the gearbox end.|
I've just thought of something I have not done - whether the noise is there when car is going backwards.
|The noise is the same going backwards - very pronounced. Can really hear the noise with engine turned off, in neutral - makes no difference if clutch is engaged or disengaged. In fact I would say the noise is not coming from the gearbox - it is further forward from the engine bay/front wheels area. |
|If you can hear it while rolling with the engine stopped and its forwards then it can only be wheel bearings. Have you spun the wheels by hand?|
|Hi Paul, yes I've spun everything there is to spin which I can get my hands on. The wheels spin with no noise. |
I replaced the wheel bearings in the winter of 2011 - they have only driven a 1000 km. I did think it could be the back plates behind the brake discs but no, no rubbing here. Then perhaps the brakes pads on one side or both, but seems clear and the bakes work fine, no pulling.
The noise is loud and very very distinct at very low speed either if the car is moving forwards or backwards, engine off or on, the gears engaged or in neutral, clutch depressed or not. The sound gets faster and faster as speed increases. I am obviously missing something. The sound is very much coming from the front end. Even had a passenger down in the footwell whilst driving to locate the sound. Same conclusion as me.
I've thought about the flywheel - cannot be that as noise is there when engine off. Propshaft - no, wrong end of car. Back wheels etc - no, no sound from there other than the dreaded 'diff' clunk when pulling away!
I'm going to check the bearings again and the brake discs/calipers and pads again.
Any other suggestions appreciated - it is driving me a bit potty
|The bearings/hubs/discs may well make a different noise when loaded, difficult to check, although maybe someone hanging out of another car alongside may be able to hear something from one more than the other. Other than that, disconnecting the propshaft and towing the car would completely eliminate the gearbox, and the prop-shaft itself if disconnected at the diff.|
|Maybe get you local friendly garage to put the front end on a rolling road and have a listen from outside the car?|
|In addition to above very good ideas, I wonder, could it be steering arm sockets (ends)contacting wheel balance weights, or the wheel inner surface itself if there is some irregularity in its 'trueness'?|
|Not had time yet to test all the suggestions - so will have to wait til the weekend - will come back with an update.|
This thread was discussed between 08/06/2013 and 26/06/2013
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