The noise only happens on heating only, when the boiler thermo stat reaches temperature, the noise is a loud droning noise and the boiler shudders.

Alfred, April 2008

Could be a partial blockage in the heatexchanger, may need replacing, if the water in your system is black/ish the whole system may need cleaning & flushing also, if you replace the heatexchanger and not clean the system it will fail twice as quick as the first!

T, April 2008