Yes its definitely caused by the pump not running or maybe only running intermittently. My pump had a loose connection to it and so it worked sometimes and sometimes didn't. Having re-fitted the wires into the connector block it ran fine and almost silent boiler again.
You can check for this easily by gently moving the cable where it goes into the pump connector box - if the pump starts and stops - thats the problem.
If that doesn't cure it - try gently tapping the pump with a soft hammer to just shake it into life.
If that fails, there is sometimes a little plastic cap at the back end of the pump - undo this and check inside if there is a square or slotted shaft inside. In either case, try moving the shaft carefully with the tip of a screwdriver to get the pump started.
If that is unsuccessful, sorry - new pump.
Ade
December 2006