You need to figure out how many cubic feet are in the space that you want to exhaust. Your space is 5'x8' that is 40 sq ft. So for every foot of height add 40 to find out your cubic space. Take into account the 30' of ducting the air will flow through as well.

Example: if your space is 8' in height you have 320 cubic feet. With a six inch 400 cfm inline fan, you should be ok.

There are many ways to dampen the sound of a fan, build a box with insulation, place your fan away from the space and people, wrap your blower in insulation, you need to find what works for you.

And the best way to battle temps is with fresh new air, especially air conditioned air. Find a way to get new fresh outside air into your grow space and temps won't be as hard to battle.