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Gearbox rattle when idling

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  • Gearbox rattle when idling

    I need an opinion about a new noise that developed last weekend (Zetec ZX1 with quite new semi-close T9).

    When I have the engine idling slowly (about 800 rpm) there is a distinct and rather loud metallic rattling from the gearbox. Imagine the harsh sound of an old diesel engine idling cold.

    When I press (disengage) the cluch it vanishes immediately. Release the clutch and it is back.

    Raising the idle speed reduces the noise until it goes away by approx. 1200 rpm (or maybe the exhaust noise drowns it).

    The only thing I know of that is moving additionally when releasing the clutch is the input shaft and related cogwheels. Something serious to rattle there?



  • #2


    I've had a throwout bearing rattle in neutral, the noise going away when the clutch pedal is depressed. The bearing was not retracting away from the pressure plate completely.

    Can you hear the noise with the clutch engaged and the car moving?

    Did you put in a new clutch, pressure plate, and throwout bearing when you put in the new transmission?

    Do you have an annular clutch or does it have an external slave cylinder and clutch release arm? If external is there any adjustement?



    • #3
      Never mind, problem solved. I learned from other sources that the T9 is prone to layshaft rattling when idling very slow and rough. And, indeed, tweaking the carbs for better synchronization (=smoother idle) and increasing idle speed made the rattle go away.

      My theory is also that the hot weather last weekend (=thinner gearbox oil) made the gear rattling only worse.



      • #4
        Hi Doug, thanks, just beat me by 2 minutes.....

        Anyway, I made (hopefully) sure that I did not have the throwout bearing jumping around. At least I installed a spring to return the slave cylinder to idle position. Also, to make the rattle go away I had to depress the clutch fully. Just pushing/engaging the throwout slightly did not make a difference.

        When the new tranny came in last year all the clutch parts were replaced and 4 weeks ago I also changed the clutch plate agin when it got oily.

        But as written before the explanation with the layshaft makes some sense. If the idle is rough and especially with lightened flywheel the changing acceleration may cause the gears to clunk within their play.