how can I fix hot water/heating problem?

I have a wm 51/3 rs baxi boiler, when we turn hot water and heating on (both do not work), the boiler comes on, but after a minute or so starts to make whistling sound and then knocking, before it switches off. I was told by a plumber that it is the circulation pump, which has been replaced, but still having same problem,no heating/hot water.Can you tell me what the problem can be?