Hi, I know very little about plumbing.

I live in an apt building with a hydronic heating system. There is a clicking noise from the pipes getting hotter. Sometimes the clicking is very rapid, almost like a machine gun (but not that loud), which will happen once every hour or so; other times, there is a single "click" and then five seconds of silence and then another single click. This happens very often.

Do you know why the two different noises would happen?

Is it possible for the clicking noise to be caused by trapped air, or anything else besides the pipes expanding?