Why does my central heating boiler fan become so noisy while the boiler is running?

Well the reason why the fan makes a loud noise is because there is a demand of heating or hot water. The bearings in the fan are slowly wearing out, however sometimes applying a little lubricant can solve it. Most of the time the fan will need replacing because theses days manufacturers will design the product to be replaced only and not to be repaired. Southstar Heating & Plumbing