I have the same case and experienced the same problem. I tried 120mm and 140mm Noctua fans (all of my fans are Noctuas), several different thicknesses of washers and gaskets (up to 1" stand-off), with and without shrouds - you can moderate the buzz, but the lengths you have to go to eliminate it defeats the purpose of having a fan there in the first place.
I gave up. I replaced the side vent cover, put 140mm NF-A14s in both front slots, rear and top-rear, and high-flow PCI slot covers in back. With that setup, I'm able to keep two non-reference GTX 770 OC GPUs under 70C during benchmarking (68C on the hotter card) with the GPU fans at 60% and case fans at about 80% which is still quiet - just air whooshing sound. Even with the case fans maxed, its louder but not an unpleasant sound - again, just the air whoosh.