Any time you have a grinding noise from the driveline of a car, it’s time to have it checked out. That’s because continuing to drive it in that condition might cause more damage than was already there. Grinding sounds from the area of the transmission can indicate worn bearings in the gearbox which, if caught early enough can sometimes be replaced without resorting to a complete transmission replacement. Even so, it will be a big job as the gearbox will need to be removed and pulled apart.

You may also find while you’re inside the transmission that there’s other wear that needs to be addressed, at which point the decision becomes whether to repair or replace. You can buy a reconditioned transmission from a specialist shop, or a second-hand one from what was once called a wrecking yard. If you do the latter, make sure the used gearbox has been tested and comes with some sort of warranty. The cost of a replacement could be anywhere from $1000 to $10,000 depending on what you buy and where you buy it from.

Any time a diesel engine produces black smoke, it’s doing so because too much fuel is entering the engine. The danger then is that the engine will start to run hot and that can cause all sorts of meltdowns. But combined with a knocking noise, black smoke can also signal serious internal engine damage.

The best advice is to not drive it any farther and get a diesel specialist to look at it. Diesel engines often make a knocking noise as a normal part of their operation, but if this noise has changed, then something’s wrong.

There’s a bit to unpack here. The first is that the D-Max is rated to safely tow 3.5 tonnes, so it’s not as if you constantly have the vehicle overloaded in terms of what it’s towing.

But if you also have the tray full of gear as well as 2.3 tonnes of caravan on the hitch, you might find that you’ve been inadvertently running overloaded for most of the time. It happens more than you think. At which point, the Pedders specialist is right; you need a suspension upgrade.

Even then, however, you still need to stay within the manufacturer’s maximum GCM (Gross Combination Mass) and the GVM (Gross Vehicle Mass) to stay legal. You can sometimes officially upgrade the GVM with a suspension upgrade that’s been properly engineered, tested and approved (which Pedders is a specialist in) but the GCM is generally not able to be upgraded according to the law in each state and territory.

But none of this explains why a vehicle designed to two 3.5 tonnes is wearing out its rear springs with 2.3 tonnes hitched up. Is the tow-ball down-weight within the legal limit (usually 10 per cent of the weight of the towed load)? Incorrect loading in this manner could affect the springs. Perhaps a fully-loaded visit to an accredited weighbridge might net more answers.

However, if it’s a simple case of the springs wearing out prematurely, then it’s possibly Isuzu’s problem to fix under warranty. Perhaps there was a bad batch of springs fitted at the factory that is affecting your vehicle and others built at the same time. Stranger things have happened.