Trying to find ways to lessen the fan noise on my Tevo Flash without resorting to major renovations, I designed a new fan adapter to fit a 40mm (4010) fan and a matching duct. This certainly improves things, but to be honest I was hoping for better. I need to find a nice quiet 24v 4010 fan as the one I have is just a cheapie, but it's still better than the stock blower.
Very easy to install, you just need 4 x 12mm m3 bolts and matching nuts to mount the fan to the adapter. Then mount the duct inside the cover and fix both using the stock m4 nuts and bolts.
I also recommend you remove that fan grille from the cover for quieter airflow. Even with the stock fan this makes a noticeable improvement. I just cut mine out with wire-cutters and filed down the edges with a dremel (or round file).