Hydronic water systems problems

I have a very weird problem in my forced hot water system with a propane furnace. One of the zones (of 6 including a boilermate water heater) makes a lot of noise like water hammer and clammering and vibration. This occurs very sporadically. But when this zone is calling for heat and the furnace is cycling. I get this tremendous noise. It only occurs when this zone is calling for heat and only occurs when the furnace is firing. I have bled the system a couple of times now by closing all valves in the system, opening the boiler feed, zone return and supply valves and running a hose off at the end of the zone water circuitin this particular zone. I still get what I think is air in the system. Can someone explain to me how air can get into the system? Could I have a faulty bleed valve? faulty expansion tank? What are the symptoms of such faulty devices? PLease help if you can...