I think I have air in my rads?

I have a glow worn ultimate... It is not a combi it is the older type will a tank in a cuboard were it stores the hot water after it has been heated by the gas boiler in the kichen. But recently I can hear air going throw the pump and some of the rads are cold at the bottom. They have bleed valves at the top and the bottom and I do not no which ones to use as in the past when I had a burst vrad pipe the system bled its self after I had repaired it.