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MG MGB Technical - Whirring sound when clutch disengaged

When I have the clutch pedal released I hear a whirring sound. When the pedal is depressed the sound goes away. The sound increases when I rev the engine. Except for the noise the MG runs great.

Other than turning the radio up or wearing ear-muffs, what needs to be done to repair the problem?

1970 MGBGT overdrive

Lee

Lee. "... a whirring sound." is not a lot to go on. To me, a "whirring sound" is what the wings of a dove make as it takes off in flight. Others have used the sound to describe a blender in use or an airplane propeller. All very different sounds.

You need to identify where the sound is coming from as exactly as possible. The use of either a mechanic's stethoscope, or in this case a regular stethoscope, should help.

Put the car up on jack stands and make sure it is properly supported. Have an assistant start the engine and let it run in neutral. See if you can identify where the sound is coming from by using the stethoscope.

The areas I can see would be the crankshaft (because it could be moved forwards when you depress the clutch pedal), the flywheel (could be loose and depressing the clutch pedal would tend to force it against the crankshaft), the pilot bushing (depressing clutch pedal would change the relationship between movement of the crankshaft and input shaft), clutch plate (slipping?), pressure plate (loose?), throw out bearing (badly worn?) and input shaft/first motion shaft.

It is also possible to do a little "advanced" trouble shooting by removing the rubber boot between the clutch fork and transmission housing and shining a light in there to see if you can gain any understanding of what is happening. It would also be possible to disconnect the clutch fork and the slave cylinder pushrod and move the outside portion of the clutch fork slightly forwards, removing the carbon throw out bearing from contact with the pressure plate.

But, in the final analysis, this is going to be an engine out inspection to correct the problem, perhaps even to find out what the problem really is.

Les
Les Bengtson

Before stethoscopes, people put a board on various engine parts and their ear on the other.
Dan Robinson

On a racing GT, we are getting a "funny noise" when the clutch pedal is depressed. More of an intermtent howl than a screech. No problems racing and seems to go away once the engine is warm. We had it once before but since then have had the gearbox rebuilt and fitted anew thrust race . The bush in the end of the crankshaft has been checked and greased with graphite grease. When the gearbox was last out for the rebuild, the spigot on the end of the input shaft that runs in the bush was very dry and we thought that was the original problem but why has it recurred?

Any suggestion swould be appreciated.
R L Tinkler

Are you used a roller clutch release bearing? If you are they can cause the noise you describe when the rollers are dry or pitted.

Clifton
Clifton Gordon

My response is for R L Tinkler.

Clifton

Clifton Gordon

Lee, a "whir" as you describe is likely from the input shaft bearing on the transmission and is common(ish). You decide when it is "over the limit" and needs to be delt with. Mine has been "whirring" for 20 years,... no change in intensity or smoothness but I will put a new bearing in next time the motor is out (it hasn't been out for 20 years...).
Rick

The whirring sound I am describing is a deep rumm, rumm, rumm. I have not had a chance to get it up on stands yet. I am certain it is not from my standard carbon type throw out bearing. It would make noise when the clutch is depressed. I am leaning more toward the input shaft bearing. If I don't replace it, how much damage can it do?
Lee

Only you can make the call. A "whir"?... as I said. A "deep rumm, rumm, rumm"?... especially in an overdrive,... I think I would get right on it! Non-overdrives are cheap and plentiful but overdrives are expensive and sought after so I would recommend that you deal with it now.
Rick

I'd say 'whirring' is normal and is just the first motion shaft spinning. 'rumm rumm rumm' sounds more like a ball bearing wearing out on that shaft. This is if it happens with the car stationary and out of gear, just pushing the clutch pedal in and out.

Noises when the car is under way could be any bearings etc. in the gearbox, OD, prop-shaft, diff, and wheel bearings. But with the exception of the wheel bearings whilst pushing the clutch in and out may change them, a better test is gradually moving from acceleration, through steady speed, to deceleration and back again. If the noise comes and goes then it is one of those bearings. If the pitch or cadence only changes with road speed it is probably prop-shaft or diff. If the noise is the same at a given engine revs, regardless of gear and hence road speed, more likely the gearbox. If it comes and goes as OD is switched in and out then it is more likely to be OD.

Wheel bearings will make a constant noise while moving, which only varies in pitch or cadence with road speed.
Paul Hunt 2

I am constantly amazed at the knowledge and information on this site. When reading Lee's original post I immediately assumed that the gearbox input shaft bearing was the culprit and would have looked no further. However, many other possibilities have been mentioned and all seem possible. Good work guys!
J C Weidner

The sound is dependent on engine RPM and clutch position. Coasting down hill with engine on and transmission in neutral with clutch out is the same as standing still at 1000 RPM with the clutch out. Depress clutch sound goes away.
Lee

Sounds like input shaft bearing, and it must be louder than normal if you notice it while the car is moving. I notice it when sitting still, but not otherwise. At least, it sounds a lot different when under load.

Anyway, whirring to me sounds like a pretty normal condition. Maybe some fancy tranny oil or additive would make it less noticable?
Tom

Oh, yeah. Common descriptions of operation are that the clutch is disengaged when the pedal is down, as in the clutch isn't engaged with the flywheel.
Tom

I have yet to find a first motion bearing that was so worn it made a whirring noise. BUT I have repaired many gearboxes where the mainshaft spigot bearing has failed. This means the complete dismantling of the gearbox, fitting a sleeve to the front of the mainshaft & a new roller bearing. There is a Timken sleeve available for this job IR15 - sizes = 15 X 18 X 25 mm long. Non O/D boxes are usually worn by 90,000 & O/D boxes by 120000 miles. I recently had one just starting to fail at 45,000. Always check when rebuilding a motor!!.
Garth
Garth Bagnall

This thread was discussed between 26/05/2007 and 31/05/2007

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