Definition:
This is the pinking or rattling noise sometimes heard during acceleration that indicates detonation is occurring inside the engine (See Detonation). Spark knock can be caused by a variety of things including using low octane fuel, over-advanced ignition timing, too much compression (often due to a buildup of carbon in the combustion chamber), by an inoperative EGR valve, and/or by too much heat. If switching to a higher octane fuel doesn't cure the problem, the cause should be investigated because prolonged or heavy knocking can damage the engine.
This just a speculative answer from me, as without hearing your car, I'm only guessing.
