Hi, I'm new to the forum as a poster but have browsed for ages so here goes.....I have a grundfos super selectric CH pump that has started knocking when it shuts off. It pumps ok and is quiet when in operation but when the boiler shuts off, it knocks loudly, sometimes for while. This doesn't happen everytime it shuts off but occasionally. The pump is less than a year old....any ideas would be very welcome, Thanks, Mark.