The 'clunk' or 'thud' on pick-up
A sharp clunk when you press the accelerator, when you lift off, or when switching between forward and reverse: this is play in the drivetrain that snaps back into place the moment the load changes direction. Several components can be responsible, ranging from the straightforward to the more serious.
Likely causes, from the simplest to the most serious
- Universal joints, flex discs (flectors) or CV joints on worn driveshafts. Straightforward repairs.
- Engine mounts, gearbox mounts or axle mounts that have deteriorated, along with the rear subframe rubber bushes: the whole assembly shifts and knocks.
- Excessive play in the crown wheel and pinion or the differential bearings.
- Worn planet gears or sun gears inside the differential.
Is it safe to keep driving?
Generally yes, but the play will worsen and every clunk puts a little more strain on the components. A badly worn universal joint can eventually break and release the driveshaft: if vibrations appear alongside the clunking, the car needs to be checked promptly.
Repair or replace?
Universal joints, flex discs, CV joints and mounts can all be replaced without touching the axle. If the play is coming from inside the differential and exceeds the manufacturer's tolerances, a reconditioned axle or differential on an exchange basis is generally a safer option than a rebuild, as setting up the crown wheel and pinion correctly requires specialist tooling.
