The GT's are very nice, but they're no magic bullet. Ultimately, if you want a quieter fan, it needs to spin slower. You might consider the 800rpm variant of the Scythe S-Flex series, which is quite nice. I'm a fan on the Noctua's as well, and the S-series comes with resistors to lower fan speed as low as 900rpm.
Are the fans quieter when they're off the rad? Some fans get noisy when their blades are near restrictions, which a shroud can help fix. Other times, the fan just vibrates the entire radiator if it isn't mounted very well. Another possibility is that the bearings on the YL's are going, which happens now and then.
__________________ i7 2600K | ASUS Maximus IV GENE-Z | GTX Titan | Corsair DDR3-2133
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