You could contact ASUS for a replacement fan.
Or oil the fan.
Some have replaced it with a Termaltake - Blue orb.
From a newsgroup:
Some useful information in case anyone else is having fan problems. I
have the Asus AGP-V7700 GeForce2 GTS card. I used it for 5 months
before the fan started making bad noise. I replaced it with a radio
shack fan and heat sink that worked fine, but, it was a 2 wire fan and
did not allow me to use the software called "Smart Doctor" that came
with the card. This software monitors the fans RPM's and chip temp as
well as input voltages. The software will shut down the card if
detects an overheat condition or voltage out of tolerance.
So, I ordered the Blue Orb from thermaltake, as it was supposed to be a
direct replacment for this card. When the fan arrived I looked it over
and decided it would fit on the card, but the power connector was way
to big. They want you to plug it into the motherboard or one of your
power supply connectors. Since I wanted to use the "Smart Doctor"
software to protect my card I needed to be able to plug the fan into
the card itself. Luckily I still had the original fan. After cutting
the power connector off and soldering it to the Blue Orb, the fan
mounted perfectly and everything is working as it should.
I would recommend the Blue Orb fan with the knowledge that you should
keep the original fan and wiring. The fan does an excellent job of
cooling the chip.