alpha 280e problem?

hello guys . this boiler has started screaming when the heating fires up ,and when hot tap ran . took cover off checked the diverter valve pistons in and out as norm. looked at diaphram all ok . checked pump and fan,ok. now boiler has gone off . boiler fires up in heating mode only if i press micoswitch on dv, fan runs as norm but noise still there , hot water does not function even if switch pressed . swaped switches still nothing. while i removed cover to burner the heat ex is a little furry . can some one advise me on which brush to clean ext if ness . and how to clean int . any help would be appreciated . cheers