Some noises you will struggle to stop and can be heard through a single layer brick wall fairly easy.
Here, i'll link you this, its the 6" version but check the data sheet so you can size accordingly with what you need airflow wise, as far as I know the same company sells all the sizes.
Quietest fan ive ever owned by a million miles and ive had some super expensive supposedly silent fans..
Check out the DB rating on the data sheet.
Find many great new & used options and get the best deals for Hydroponics Fox Carbon Air Filter Twin Speed Fan and Ducting Kit 100mm 4" at the best online prices at eBay! Free delivery for many products!
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If you cant find the size you want, try this shop
Acoustic Low Noise Silent Inline Box Extractor Fan. Consisting of a centrifugal fan mounted inline, in an acoustically lined box with circular duct spigots for inlet and outlet connections. In Line Fan is quiet running external rotor motor with thermal overload protection.
www.ebay.co.uk
which is where I got both mine.
Combine that with a variac speed controller and its almost silent.
Loudest thing in my room is my circulation fan.
Do not turn off your extraction ever unless you want your neighbours to smell your grow!