A dull thump somewhere under the floor at exactly the moment you lift off the throttle and then get back on it is almost always not about a breakdown but about accumulated clearances. The torque changes direction, the parts move within their play and strike one another. Here is where that clearance comes from, which parts give this knock, and where to start checking so you do not replace everything in turn.
Why the sound is tied to the change of load
From the engine to the wheels the torque passes through a dozen connections: gearbox gears, splines, driveshaft joints, rubber mounts. Each has a small design clearance — without it the parts would seize from heat and deformation. While the car is driving under throttle, the whole set is pressed in one direction: the teeth bear on their drive faces, the splines are taken up to the stop. Lift off, and the force changes sign — now the wheels are pushing the engine. Every clearance is passed right through, and at the end of the travel the parts meet with an impact.
A single clearance of hundredths of a millimetre is inaudible. The sum of a dozen worn connections gives a distinct clunk. Hence the portrait of the sound: one or two dull thumps rather than a series; strictly at the moment of transition; most often in traffic, where throttle and brake alternate every few seconds. At a steady speed that knock is silent.
Which parts give this knock
| Part | Additional sign |
|---|---|
| Driveshaft splines, inner CV joint | Impact closer to one of the front wheels |
| Outer CV joint | Beyond the thump — clicking in a tight turn under power |
| Propshaft universal joint or splines | Sound in the centre of the floor, on rear and all-wheel drive |
| Propshaft centre bearing | Vibration at 60–90 km/h joins the knock |
| Engine and gearbox mounts | A jolt through the body, the engine visibly nods |
| Backlash in the final drive, differential | Dull thump at the rear, often with a hum at speed |
The individual portraits are in signs of a worn inner CV joint, signs of a worn outer CV joint, signs of a worn propshaft universal joint, signs of a worn propshaft centre bearing, signs of worn driveshaft splines and signs of worn engine mounts.
The mundane possibilities deserve ruling out separately: a subframe left under-torqued after a repair, worn trailing arm bushes, a loose exhaust clamp. They sound almost the same and cost incomparably less — noise underneath: exhaust or undertray.
Narrowing it down over a couple of drives
The most informative technique is to compare how the sound behaves under load and without it. Accelerate to the speed where the knock repeats and try the same manoeuvres with the gearbox in neutral: if the thump has gone, the source is in the drivetrain; if it stays, it is in the suspension or the exhaust.
The second guide is the reaction to loading. Driveline play barely depends on the weight of the car, while the suspension knocks noticeably more readily with four passengers; that case is covered separately in a knock only with passengers or a load. The third is where exactly you hear the impact: under the driver’s feet, in the centre or at the rear. The full set of similar sounds is on the symptom page knocking when accelerating.
How the repair is decided
Half of these cases close with tightening fasteners and replacing one mount; the other half come down to the driveshafts or the propshaft. That is exactly why the diagnosis is worth paying for separately, and why “replacing the whole assembly” should not be agreed to without being shown the play. Ask the mechanic to move each joint by hand on the lift with you watching — play in a CV joint and in a universal joint is obvious once you see it.
Two things mechanics do not always say. A CV joint with an intact boot and early clicking is often rebuilt rather than replaced — new grease and a new boot for a fraction of the cost, and it is worth asking whether the joint is in a condition for that. And a torn boot found now, before the clicking starts, is the cheapest repair on this whole list: a torn CV boot: how far can you drive.
If the knock is still rare and you are not sure it is even there, record a town drive in the Pro-Stuk app: the most useful stretch is one with several accelerations and stops in a row. The app matches the recording with your answers about when exactly the thump occurs and shows the likely causes with percentages, so the conversation at the workshop starts with a specific part.
