On 28/01/2020 22:18, Jimk wrote:
Does it have "back draft shutters" fitted?
They are thin light plastic spring loaded flaps (usually a pair,
each semi circular) usually mounted immediately before the
ducting connects to the fan output spigot.

They are pushed open by airflow from the fan being on, then when
fan is off they shut due to their springs. You usually hear them
as they "slap" shut after the fan is switched off...

And constantly slapping open and shut when we have strong gusty winds :-