I have a FHA furnace serving a 2,400 square foot 2 story house. The furthest bedroom on the second floor was a bit cold and when I checked it was the last take-off on the main trunk line in the basement. To get the room a bit more air, I installed an inline Fantech fan. The problem that I am having is that the ductwork is now noisy. The configuration is as follows. Sheet metal trunk line in basement with a 7" diameter take-off to the second floor. I installed the fan on the 7" line in the basement. the 7" line transitions in the basement to a 3"x12" oval duct thru the first floor wall, when it reaches the second floor, it goes across the floor joists (first floor ceiling, second floor - not attic) with a 7" flex duct to the diffuser. I am hearing a lot of noise in the wall were the 3"x12" duct is. Using my ductolator, I come up with a velocity of about 950 FPM which I know is a lot. Presently I have a reostat installed on the fan and have it running about 1/2 capacity. When I turn the fan down the noise goes down but the room gets cold.
What are my options? Is the majority of the noise due to the flex duct or a combination? Any help would be appreciated.