Hi all, this is a question for someone with central heating experience.

We have a brand new central heating system (completed as part of a fairly extensive renovation project) and one of the pipes makes quite a bit of noise. It does it while the heating is on. To describe the noise:
- few minutes pause
- then a series of popping / crackling for about 5-10 seconds
- few minutes pause and then repeats

If it were dripping or something, wouldn't it do it constantly - with no pause while the heating is on and the pressure is up? I understood from the plumber that due to the joists (as this is in an upstairs bedroom) there was a tight fit for the pipes and he had to work around it. We've laid flooring down, but weren't staying in the room at the time. So we weren't aware of this constant nuisance until recently.

There are two radiators near where the sound occurs. I've attempted to bleed them many times, but no air escapes. I've even tried limiting the flow to one of the radiators and that seemed to help for a little while, but not anymore. The towel heater in the bathroom needs a little bleeding every few weeks. But this may be because a lot of air was in the system when it got installed and we've taken radiators on and off to install the flooring. But if this were an air issue, wouldn't it move on eventually?

Any advice is appreciated. Our setup is brand new, combi boiler, no tank (that I'm aware of), pressure is good around 1.5 (just shy of 2 bars when boiler is at full blast).

Thanks!