Potterton FSB HE 30 new July 2009?

Have a howling noise after firing up of boiler. Have had 4 visits from BG...1 new fan fitted, no joy. Also on fire up a slight whoomp? they checked gap between ignition thingie..ok. All rads heat up. Pump next to HW cylinder set on highest setting, turned this slowly down to the lowest setting and the noise diminished, but on the lowest setting the pump started knocking and noisy, so turned quickly back to original setting. Two rads nearest the cylinder need bleeding quite regularly. BG have suggested Power Flush, I baulked at the price, and they then suggested going to a local CH Co. they could do it cheaper... Late last year a new gas meter was fitted. Gas bill also arrived...consumption, as expected... up, but by 125% (there's a comparison graph on the bill) seems excessive to me? Thanks in anticipation :-)