Boiler - sound insulation?

Box it in.Refer to the boiler manual about what size gaps all round the boiler you need as each boiler will have different regs.Do not insulate it as it will cut the circulation and be dangerous and cause the boiler to fail.Be sure to have a access panel under boiler for maintenance.By boxing it in will reduce the noise considerably.Final note if its a loud humming noise you probably need your system flushed.Many years experience.