water circulating into expansion tank?

I have known (rarely) the coil in the cylinder to have corroded and split and have the effects that you describe.

ED.
January 2009

pipework probably blocked where the open vent and cold feed pipes meet. If you run a magnet along the pipework by the pump and it sticks of its own accord then its got a build up of sludge that has to be cleared. Best way to clear is drain the system and remake the blocked section

andy
January 2009

does the pump have several speeds?.use the slowest setting and see what happens

a1
January 2009