Pinking / knocking noise under acceleration

A thin, sharp metallic sound, like 'a handful of marbles shaken in a tin can,' shows up especially uphill, under full load, and mostly at low revs. This noise, called knocking (or 'pinging'), is a sign of pre-ignition: the air-fuel mixture is igniting on its own, too early, from the heat and pressure, instead of waiting for the spark plug's spark. These multiple flame fronts collide and create pressure waves that hammer the piston: that's the sound you're hearing. Knocking isn't trivial — sustained, it overheats and can damage the piston, rings, and cylinder head — hence the importance of fixing it promptly. Causes, from simplest to most technical: fuel with too low an octane rating for the engine (the most common and easiest to fix: just refill with the correct octane); a mixture that's too lean, which raises combustion temperature (carburetion, air leak); carbon deposits in the combustion chamber, which locally raise compression and create hot spots; and too much ignition advance (ignition out of adjustment or a faulty sensor), which triggers combustion too early. The distinction is useful: knocking that shows up right after a fill-up at an unfamiliar gas station points to the fuel. Urgency: don't let knocking linger — ease off (avoid heavy loads at low revs) and fix the cause quickly, starting with the simplest and free one: the fuel.