Fer Falcon combi makes thudding banging noise!! ?

Hi all- need some advice please - my 7yr old Fer combi makes a very worrying racket when its come up to temperature. During the warm up period (20 mins approx) it is silent in operation. The pressure gets up to about 1.75 bar. As soon as the noise starts, I switch off the heating immediately! The DHW supply seems unaffected so far. Could the boiler be dangerous to use? I need a cheap as poss fix because I got laid off last week. It never rains...