Potterton Promax 15 SL?

C/Heating running 11.rads 3 convector type modern about 3yrs old rest old ribbed type.
Boiler Potterton Promax 15 SL supplied and fitted by British Gas 7th Oct 2010. Makes horrendous tapping sound on fire up for around 2 mins then faiding out to a sound like a bumble bee flying around in a confined area. What is the possible cause?.
The engineer from BG who fitted a new fan 3 days ago said the header tank water is very dirty and cloudy and yet in the fitting contract there is a charge of £80 for powercleans what is that is that what's called power wash? and is or should it be applied with a pressure sytem to flush out all ths s....t if so the cloudy tank says it's not been done or not done properly. Me thinks the power wash bit was skipped as I earlier took a rad off in the bathroom and the water that came out was fairly clean. How should this cleaning be carried out?.
Many thanks for your time reading and replies in anticipation.