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The general rule about computer fans is that the bigger they are the the quieter they are (i.e. 140mm > 120mm etc.) It's a wierd market gap that large "commercial" air moving fans are so loud.

There are continuous extraction fans, they're called dMEV systems. Eg https://www.epicair.co.uk/products/vent-axia-lo-carbon-reviv...

There's also systems which can move air both ways through the whole house - https://www.ventilation-alnor.co.uk/index/support/alnor-know...

Your DIY solution looks a lot cheaper though.

Vevor makes drop-in floor/wall register fans in the $35-40 price range. The 'proper' solution is inline booster fans in each leg of the ducts, but retrofitting can be expensive depending on access.

Houses with rads don’t have ducts or registers. There’s a whole other class of solutions for these houses.

if you have house (with attic), jump ducts are nicer solution