I keep getting air pockets in my cooling system.

I keep getting air pockets in my cooling system, they sometimes cause my car to overheat and run hot until air is bled out of the lines. Ive put in a new water pump and thermostat and the problem still occurs. The oil looks good like normal oil should no visible leaks or puddles under the car. But in the past few days, after car has been sitting for a while and I go to start it there is a terrible knocking noise that goes away after a few minutes. Ive heard that it could be intake gasket, Ive heard that it could be the head gasket. Ive heard it is blown and not blown. The car still drives well and has not lost any power so to speak. PLEASE HELP!