their could be several reasons for knocking on idle.. e.g
Press the clutch on idle, if the sound gone then it might be clutch bearing sound or simply ur tranny needs right grade oil.
It could be simply bcuz of low grade fuel, refill with high octane drive it for 10 to 15 kms .
sometimes coils, plugs and bad wires can also cause knocking....
may be the timing of ur car is advanced....