From another Ford website .......
The knocking sensor sends the information to the
ECU which then reduces the ignition advance (timing) to stop the knocking. Knocking occurrs when the explosion takes place before the piston reaches TDC (top dead center) and is quite a descructive phenomenon for engines. If not controlled is usually creates holes in pistons. If you've driven the car for any length with this noise then it's doubtful that it is actually caused by knocking - you'd have had all sorts of problems after not many miles.