Dude, I think it's due to the Noctua not having enough air going in it and not having enough space to push it out. What the hell sort of a case is that for hardware like this? Also bear in mind as that's your only exhaust that all the warmth from the GPU goes up and gets sucked straight into the heatsink. Just saying, there's some really decent cases out there for very little cash - Look at the Xigmatek range or something.