Steamhead might be on to something. That system is piped right for venting, and its piped to pump away from the exp tank. Thats all current standard practice and works very well. The benefactor of this website even has a book about it. "Pumping Away" - Dan Holohan.
Even if the piping was laid out poorly for air venting, you have manual air vents at each baseboard.
If you run just the pump, is it quiet?
Does the noise only occur when the burners are on?
If so, thats caused by a layer of scale build up in the bottom of the cast iron sections that get super hot, and flashes of steam are made - only fix is strong chemicals - I advise consulting a pro for sure to handle that job.
I would recommend Rydlyme as the cleaning agent.
If the noise happens all the time - it may be that the pump is too big and you here velocity noise.
Air normaly sounds like water falling inside the piping, like a trickling noise.