Pontiac Repair: grand am overheating/ knocking noise, valve cover gasket, proper mixture

Question
 I wanted to give you a little bit more info on the situation so you can make a proper diagnosis.  

The service engine soon light starting coming on right after he replaced my thermostat and the knocking noise started after I had already gotten home and parked the car.  It was sitting idle for about 2 minutes and the whole time it was a rhytmic knock about 4-5 seconds apart.  

   The overheating was seemingly under control after the thermostat was changed for 4 days then I parked the car and the coolant was leaking underneath the front passenger side on top of the tire then of course to the ground.  I could also view a small stream from underneath the very front of the passenger side of the car but in the same area of the tire.  The radiator was NOT flushed when the thermostat was changed.  They did not have time.

  The coolant only leaks when I park otherwise no leaks can be detected.  The only area that the coolant comes from is up under the radiator cap, no where else.  The fan seems to be running well.
 The last oil change they told me there was a valve cover gasket leak.  

the head gasket was leaking

Thanks

Answer
still sounds like boilover.my old explorer does this when its hot and i kill it.replacing the cap would probably remedy this.if the engine is not overheating i would not worry to much.i would also make sure the proper mixture of coolant was used to refill the cooling system.this will help with boilover.