If this problem is still open.
If you put a screwdriver to your ear and the pump - you can use it as a screwdriver.
Did you flush your system - is all the sludge gone.
Was the water black?
Did you have lots of air to bleed from the radiators.
If either is true its not air its hydrogen - your system rusts with lack of oxygen - so the oxygen is taken from the water - hydrogen is left.
The fernox should fix this in future - but you could have sludge left - hence the need to flush the black rust away.
andy
March 2009