(i) A slight but perceptible change in engine noise:

• A developing fault in a cylinder (e.g. a faulty injector, a stuck ring, or a leaking exhaust valve) causing abnormal combustion noise.
• A bearing problem (e.g. a main or big-end bearing starting to wipe) causing a knocking noise.
• A change in the fuel (e.g. water or sludge in the fuel) causing rough combustion.
• A turbocharger problem (e.g. surging or a bearing fault) causing a change in the noise.
• A piston/liner problem (e.g. scuffing) causing a change in the noise.

(ii) An alteration in engine speed:

• A governor fault or a change in the fuel supply (e.g. a blocked filter or a faulty fuel pump) causing the speed to vary.
• A change in the load (e.g. a change in the propeller or the electrical load) causing the speed to change.
• A fault in a cylinder (e.g. a misfiring cylinder) causing the engine to lose power and the speed to drop.
• A governor set-point change or a control system fault.

(iii) A change in exhaust temperature spread pattern:

• A faulty injector (over- or under-fueling a cylinder) causing that cylinder's exhaust temperature to rise or fall.
• A leaking exhaust valve or a loss of compression in a cylinder.
• A change in the fuel quality or the load distribution.
• A scavenge fire or a turbocharger problem affecting the air supply.

1. Investigate the cause: take the engine to a safe load, and investigate each change - check the exhaust temperatures of all cylinders, listen for abnormal noise, check the fuel system (filters, pumps, injectors), the governor, and the turbocharger.
2. If a faulty injector is suspected, check the injector (atomization, leak-off) and replace it if necessary.
3. If a bearing problem is suspected, check the bearing temperatures and the oil pressure; reduce the load and investigate.
4. If the fuel is contaminated (water/sludge), change to a clean fuel tank and clean the filters.
5. If a cylinder is misfiring, check the compression, the injector, and the exhaust valve; rectify the fault.
6. If the governor is faulty, check and adjust it.
7. If the turbocharger is surging or faulty, check it and rectify.
8. After rectifying the cause, restore the engine to normal load gradually, monitoring the temperatures, noise, and speed to confirm normal operation.