If it's a large house, the noise could be entirely separate from the actual boiler. For example: heating circulator pump, hot water circulator, air fan for something or other, ... . You name it!
Is the noise continuous? or intermittent? What frequency (lower than mains hum - 50Hz? higher?) Single frequency or a 'mixed' noise? A boiler fan tends to produce several different noises mixed together. Water pumps usually produce a single low-frequency hum.
Is the noise continuous? or intermittent? What frequency (lower than mains hum - 50Hz? higher?) Single frequency or a 'mixed' noise? A boiler fan tends to produce several different noises mixed together. Water pumps usually produce a single low-frequency hum.