I turned on my (combi boiler powered) heating for the first time this
season a few days ago, and the pump made one hell of a racket. I was all
for calling out a plumber when I realised I hadn't bled the radiators
yet. A quick bleed of the top radiator (I have one higher than all the
others, so all the air ends up in that one) and the pump is near-silent
again.