Jaydah. If your central heating system is in distress it is best to get a professional in. we have a contract with british gas and I would love to call them out but it seems to always work. they come once a year to service the central heating system.
We had that kind of noise once. Had to get a plumber eventually. Something to do with either corrosion or sludge build up in the pipes. He drained down the system then put in an additive and all was well. Cannot remember if it was expensive or not...couldn't have been too bad or I would have remembered.
Turn the system off, let it cool down then try to bleed the radiators for a start.
If no joy, get a man in.