The fan on my Myson apollo comes on every 5 mins for about 10secs. ?

The fan on my boiler comes on every 5mins for about 10 secs. This is all day and night, even if the heating and boiler is off. Called out gas engineers, some say its normal. others say there's something wrong but don't seem able to fix it. Its so bad that the sound of it all night is disturbing neighbors as well.