I live in a house which is about 6 yrs old I've lived in it for about 51/2 yrs and think that the central heating system has been whistling for the last 3 gradually getting worse. It is an IDEAL combi boiler I think. It whistles for between 5 - 10 seconds at a time numerous times throughout the day. It can cbe heard from outside and appears worse now it is colder. I have had British gas around they have done numerous checks serviced it changed the fan but to no avail. They say it is a clean system and appears in good working order. They did mention about trying some kind of cleaner in the system.The whistling is driving me mad any ideas as British gas have failed me.