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?
As the pipes expand and contract, if they're rubbing on something, they'll click. It will likely be quicker when the heat turns on because the temperature of the incoming water from the boiler is quite hot; when the circulator turns off, it just stops moving the water, and then, instead of introducing really hot water into cold pipes, it just sits there and slowly cools off, so it doesn't change temperature very fast, thus the slower clicks.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013