I don't see how the registered and the experienced guys on here are being of any help to people like OP, or anyone else, by not being able to suggest what possible causes are there for his droning noise, in saying so that does not mean that OP is going to start taking his boiler apart and try and fix it himself, he is only asking for your expert opinion, you are not actually authorising him to carry out his own repairs are you!
If he does, you are not liable, if he does and it goes wrong, he will be the one who will be facing the consequences, and I don't see how you guys assume that all the people who come on here are seeking your expertice to try and repair their boiler, they may just be asking what do you think it might be and they probably want to make sure that they haven't been mislead by their own RGI who may have told the owner loads of porky pies.