Airlock

Please help.
I recently replaced my radiator on my TD5 Disco II due to a leak. Filled her up ok and have now driven +/- 4,000km without any problems. Now all of a sudden it sounds like there is air in the system rushing into the heater matrix down at my left knee. It only appears to happen when she is cold and when I accelerate. There is no sound when driving normally. I have not lost any coolant (check it daily) and there has not been any indication of over heating.
Could it be possible that an airlock sat in the system for that long before showing itself. If so, how do I get rid of it.
I'm worried that the head gasket may have gone but 2 seperate sniff tests have not confirmed it

On the top hose their is a bleed screw little plastic thing you can put a flat head screw driver in it if its tight. you bleed the air out of their. Dan

The system was originally bled with the bleed screw out and as I said there were no signs of air for 4000km. How do I circulate the water with the screw out in order to get the airlock out. It seems trapped around the heater matrix as the sound always comes from there. How do I get it to move cause I would have thought it would eventually find its way to the expansion bottle and the levels would drop??

I just took the bleed srew out. put heated on max let it run hot with the srew out and it was just bowing coolant out the hole then u no theirs no air.