If the pump was the only thing changed before the noise started then it can only be pump fitted the wrong way round or a worn out 3 way valve . Could also need the bypass opening a little if there is one .
The bang is probably a worn out three way valve . When they sometimes make a bang when springing back to hot water mode when heating shuts down . That is most likely when it does it , and the flow of water through it makes the ball shut the heating port very suddenly causing a bang , if you have...
If you only have 3 radiators upstairs and two downstairs I’ll be suprised if balancing is the issue. Make sure the pump valves are fully open and if any isolation valves on the boiler which if it’s a system boiler there will be and if it’s a combination boiler there will be make sure they are...