I have a powermax boiler 155 formally made by IMI Waterheating,its about 5 years old and until recently it was whistling, every time the heating came on it started about 2 minutes after it fired up.
Over the past 6 months I tried everything de-sludging it replacing pressure relief valves but all to no avail, it finally became too much and I was sick of getting up just because the heating came on.
I finally relented and phoned Baxi (heat team)
http://www.baxi.co.uk/aftersales/contact/engineerVisit.html
Telephone: 08706 096 096.
and asked for an engineer, previous tradesmen didn't want to know!
The cost was .....get this .......£235.............
Well he turned up said I know what that is and fixed in about 2 minutes it was the air/gas mixture he turned a tuning screw and bingo fixed no more whistling, he also changed the whole valve unit just to be sure ......at least I got something for my £235 !!!!
So get your local heating engineer to find this screw and give it a turn
Good Luck hope this helps