In article ,
Owain writes:

Get a big 8" or 10" through-the-wall fan from Xpelair or VentAxia,
designed for commercial use and priced accordingly. They should run 12
hours a day for years and be almost inaudible (except for when the
backdraught shutter snaps shut).

The 4" £20 ones from the DIY shed will start squealing, or burn out and
start a fire, or cause other distressing nastinesses.

I was about to post exactly the same response.

Backdraught shutters now seem to be operated by a thermal solenoid,
so you don't get the loud bang of a magnetic solenoid anymore.
OP probably doesn't need one with shutters in any case.

I picked up a very nice 6" Vent Axia from Cash Converters for £10,
original box, full instructions, etc. It turned out to be about 10
years old, but had never been used (and was old enough to still have
the magnetic solenoid shutters). It was intended for fitting in double
glazing, but is now mounted in my garage wall, with a few minor
modifications. This model is still sold (now with thermal solenoid)
for over £100. It's certainly not inaudiable though -- it's a high
throughput model.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]