All 4 inch fans seem loud to me, the best option is to buy a 5 inch, and reduce the speed 😉 

@redeyejedi

Had the same problem in my shed, wanted it to run silent,  

after looking at all the options, there was only one that would work. 

use one size lager Acoustic ducting and filter than the fan, I built a cupboard and insulated the fan inside (it is silent, can't hear the fan) 

maybe a 1m length of 8"/10" Acoustic ducting would slide over the fan and you could tape the ends tight to the 5" ducting  (this should make the fan silent)

AC Infinity does make a 4 inch model,

AC Infinity CLOUDLINE T4

The unit is available in the USA with the auto controller for $119usd. My tents are a little smaller than yours at 36”x20”x62” and I just used 6” fans and filters. They run at very low speed and are very low noise. They are running in the background while I’m on the podcast and you cannot hear them.

I run a twin speed ram 4" inside my tent with the filter directly attached, brought the noise down alot, aqustic fibreglass filled ducting ,and i can barely hear it , I get the idea running larger gear, but My unit easily copes with a 1x1x2 think its rated around 200m2h and paired up with a fox 100 x 300 and i thought I was going ott ,if its struggling I whack it up a gear ,my osilateing fans inside the tent make more noise  

Thanks for all the replies guys, as always some great suggestions. 

@monkeydo - ive found the 4' model now but its unavailable in the UK from Amazon. On their website it looks like its free shipping to US only but i might drop them an email and just see if they do ship to the UK. 

Bigger fan is a good suggestion but im so broke after moving lol. I might have a look into the larger size Acoustic ducting idea as this would probably be the cheapest and like I said air flow is fine at the moment so if this just reduces the noise slightly then job done. I can potentially look to upgrade to a better fan in a couple of months when my finances are a bit more secure as long run id like to double my tent size and have a veg tent and flower tent going.