There’s every chance this is caused by a small amount of wear and slack in one of the joints in the driveline. If that’s the case, those joints and couplings need to be checked and the worn one(s) replaced, as continuing to drive as it is could see the driveline fail spectacularly at speed.

The reason you feel the jolt on the flat and not when you’re parked on a hill is that the driveline is relaxed on the flat, leaving a little slack for the thump to occur. When parked downhill, gravity has caused the driveline to take up that slack (plus your foot is probably on the brake, preventing the car rocking or moving at all) so the thump can’t occur and, therefore, isn’t felt.

There doesn't seem to be a huge number of similar complaints to yours, Len, but that's pretty cold comfort. You could have a problem with the transmission itself or the electronics that control it. Either way, it could be that the transmission is stuck in high gear as a form of limp-home mode to avoid doing any further damage to itself.

The first step is to have the vehicle scanned for fault codes. Sometimes, a fault will be logged that seems to have nothing to do with the symptom, but can still cause the havoc you're experiencing. Try a transmission specialist first, as it's a fair bet they will have seen this exact problem once or twice before.

Broadly speaking, diesel engines have longer life expectancies than their petrol equivalents. That's because the typical diesel has to be more strongly engineered to cope with its inherently higher compression ratio and heavier moving parts, which also means it runs slower with less stress.

However, the advent of turbocharging and higher diesel engine speeds have also seen some lighter duty diesel engines lose some of that under-stressed longevity, meaning that they're not living as long as some old-school, slow-revving, under-stressed diesels. Modern changes including DPF technology, exhaust gas recirculation (for emissions) and single-rail, high-pressure injection have also contributed to shorter working lifespans in some diesels.

In any case, the life you will get out of any engine is directly related to how well it's been serviced and maintained throughout that life. A petrol engine treated to proper servicing will easily outlast a diesel that has been neglected and abused. As for the Hyundai petrol engine in your van, the rule of thumb says you should be aiming for at least 200,000km before spending any meaningful money on repairs. Again, though, this is entirely dependent on regular maintenance.