Bleeding air from my underfloor heating manifiold?

I am having problems with my underfloor heating manifold in that everytime I start up the heating all I can hear is air rushing around the mainifold. I have auto bleed valves on the manifold but it doesn't seem to be doing the trick. I have 2 bleeds up in the loft for the flow and return to the manifold but they never need bleeding and always throw out water when checked.
I have turned off all the heating circuits from the manifold and the air noise remains the same.

wallymotors
October 2009