help：heating问题请教，先谢谢了

我的房客上个月13号发email给我：
We've been hearing a lot of bubbling in the heating pipes lately, is
there anything we could do to purge the pipes ?
Thanks,

第二天plumber 将boiler的水和气放了。

刚才又收到他们的email
We just want to warn you that even if someone came for the heating pipes, it's still making a lot of bubbling and a lot of clicking (it's getting worst).
Do you think it's normal?

Boiler是烧热水的，己用三年，一共装了十二套，第一次听到有这情况。

请问各位大拿什么原因?如何处理?

装的人去度假了，不知道Handyman能否处理！

谢谢