whats happening is the fuel mix is igniting itself,before the spark from the spark plug.this means the piston is still coming up the bore when it meets the explosion.what you hearing
is the cylinder walls distorting.pre-ignition (pinging/pinking) can split bores & break pistons,so if you hear it,lift your foot.it's usually caused by too big a load in too high a gear,like an auto that wont kick down.putting the timing back to standard will fix the problem.