Very noisy boiler, neighbours complain.Got hot water.Radiators not so hot?

Hi,

how do you fix a very noisy boiler.The neighbours in the adjacent flat have complained about it.There is hot water.But the radiators are not so hot.The gas safe engineer on phone mentioned it sounds like the problem is coming from the combustion chamber. I would like to be knowledgeable about this when he comes to check it, so that i don't get charged a fortune.Thanks.