baseboard heat noise
Are you sure the noise is air in the line, rather than pipe expanding/contracting as it heats and cools or simple water turbulance?

Is there a manual bleeder valve you could use to release any trapped air, as there is on older radiators? If so, there are automatic bleeders (basically float valves) which could be installed. You'd want to do that at the highest point in the loop, since that's where any air will wind up.

My house also has an device installed at the boiler end of things which lets bubbles escape from the system there, before they can circulate farther.