The TT's narrow slats on the exhaust ports do generate a lot of turbulence and so noise. When I swapped out the old fans, I replace them with slimline ones, and set them back from the edge of the case. Basically, the closer the fan blades are to the slats on the exhaust ports, the interference there is to the airflow, and so more noise. Getting them further apart smooths things out a bit and makes life quieter.
I ordered two cheap fans as well of the same dimensions and gutted them, leaving just the main frame, and used it as a duct and spacer between the case edge and the new fans (creating a little "tunnel" for the air). It has reduced noise a lot. As well as being slimline, I also chose new fans with small central motors. I settled on these:http://www.ebuyer.com/124543-startech-6 ... -fan6x1tx3
Another thing to both improve airflow and reduce noise would be to remove the slats on the exhaust ports with a dremmel cutter, and replace it with a less restrictive grill like this: