Well I asked AI most likely causes:- for what it's worth, the clunk seems t be a common problem across the CX range and SEEMS TO comes on with wear.......eg I never heard a clunk when I acquired my car, now 8 months on and a further 2200 miles mostly town driving and I get an increasingly more frequent occurance of a clunk
SO AI CAME UP WITH THIS, its a starting point......also see my post under P4 siuspension which also refers to the diff pinion bolt losing its factory torque settings.
Standard Drivetrain Backlash (Normal Characteristic)The BMW-sourced ZF 8-speed automatic gearbox and open differential have inherent design clearances (backlash) between the gear teeth to allow for thermal expansion.
The Cause:
In a heavy BMW car, rubber isolation bushings and thick sound-deadening absorb the mechanical take-up. In a raw, lightweight Morgan, you hear every millimeter of that free-play taking up slack as the gears mesh under load.
The Test:
With the car safely off, you can verify this by holding the differential's input flange still and minutely rotating a rear wheel back and forth. A tiny fraction of play is normal, but it should not feel entirely loose or sound violently metallic.
Worn Driveshaft U-Joints or CV JointsThe shafts transferring power from the transmission to the differential, and from the differential out to the wheels, rely on joints to flex.The Cause: If a Universal Joint (U-joint) or Constant Velocity (CV) joint develops play, it will create a hard click or clunk right as you take off
.The Fix:
Inspect the rubber boots on the half-shafts for tears or leaked grease. If the grease has escaped, the joint will wear rapidly and cause a clunk under load
Rubber Suspension and Subframe Bushings
The Cause: T
he differential and rear suspension are bolted to the chassis via rubber rubber mounts. If a bolt has worked slightly loose or a bushing has degraded prematurely, the torque of the BMW B58 engine will twist the mechanical assembly, causing it to physically slam against the frame.
The Fix:
Inspect the tightness of the rear suspension control arms and the differential mounting brackets to ensure nothing is shifting under load.
