its has a horrible heatsink, I owned it (also read the reviews). Sounds like you stuffed a vacuum into your PC with the fan running at 30%.
I even flashed the BIOS as ASUS suggested to fix it, didn't work. The only thing that worked was setting the fan to 10% and it still made noise. Check out the two GTS250 I linked.
I forgot where I saw it, but someone upgraded from that to a GTX 480, and it was louder than the GTX 480 at 100% fan speed.
Possible he had a defective fan like you......
( A hig percentagae of those cards have fualty fans, something about the ball bearings in the fan or soemthing_