In article ,
"Steve" says...
I have 3 extractor fans in my house - 2 have no backdraught prevention
and therefore let a lot of cold air in, especially when the wind is in
the wrong direction. One had air operated shutters in the duct but they
flapped about noisily in windy weather, so I changed them for an outer
grille with air operated flaps. That makes a racket too.

Does anybody know of a domestic 4" or 6" extractor fan for wall fitting
that has effective, silent backdraught prevention?

This one seems to have a solenoid-operated shutter:

http://www.tlc-direct.co.uk/Products/BGGXC6.html