Good evening, this is most likely to be the heating pump which pumps water around the radiators.
For hot water the pump doesnt need to run as the pressure from the mains water pushes the water through the boiler heating chamber and out through the tap.
In order to replace or service the pump your going to need to arrange for a gas safe engineer to attend, if you would like me to recommend an engineer in your area please provide me with your postcode.
Pumps can be expensive to replace they often cost over £100, such as the one here, however by getting an engineer to attend they can prove it is the pump at fault, by making it run with the rest of the boiler isolated, they can also look to service it and see if that stops the noise
Please let me know if I can be of any further assistance.
I havent heard back from you please let me know if I can be of any further assistance?
Should you have everything you need please rate the service using the star rating on this page.
So I can close the case I would appreciate it, if you can rate the service received using the star rating on the question page.