Cheapskate heating "engineers" often omit one of the basic essentials, corrosion inhibitor. To be fair, sometimes it can be drained out and not replaced. Either way, lack of it means continual short and long-term problems. One of these is a noise called "kettling", you can guess why!
I recommend you find out whether you have inhibitor in your system, quickly. The most common long-term problem is black sludge building up in the boiler waterways and radiators, eventually blocking something.
Colin, January 2010
