Why are we getting air in our boiler pipes?

We installed wood floors this summer. They went under our radiator pipes. (We have base board heating) When we turned our boiler on this season, there was a lot of gurgling, way more than normal, at the beginning. Then, one area would start to make what sounds like spitting and hissing noises. We called a plumber, but he insisted that we only had air in the pipes and to just keep adding water and draining off the air. The only thing is, we can add a lot of water then bleed it off, but it comes back, and the spitting noise starts up. Is it possible that we DO have a broken pipe?