recentley serviced& power washed system so ok except for an erratic worrying loud fast drumming sound ending in a clonk! only on boiler lighting.once in every 1 to 20 lightings very erratic! have bled system many times to no avail. last plumber assures me all is well! is it?

Malcolm Hobbs, February 2008

sounds like your system is airlocked. Try connecting garden hose to drain point on system and 'backfilling' via mains cold water supply for approx 1 minute.

Curtis Lee Patrick, February 2008