Have any of you guys ever experience a knock at a hot start intermittently?

I've experience this about 5 times within the past 7 months.

Turn off my car coming home from work; wash up...10mins later; when starting the car up again; it would turn over but not start. Second attempt i would hear one audible knock and the motor starts up.

Don't really know where i should be starting; if it was the fuel pump not holding pressure in the fuel system; it would be persistant.

My only other guess would be ignition timing; but its electronically controlled..

Any other ideas?