I also have a Quicksliver that was very noisy when I got it.Progressively louder and louder as it heated up and the fans gunned up.
The choices as I saw it were:
-get rid of the Quicksilver
-quiet it down.
So, first I replaced the fan in the Power Supply with a Thermaltake 80mm fan.Huge improvement in noise, air coming out still same temp.
The case fan I replaced with a 120mm model ( forget the brand)with the option of running it at 12,9 or 7.5 volts.Massive improvement in noise again. I normally run it at 9v and turn it up to full speed when it's been working hard for a while.You can buy a power adaptor at Radio Shack that will switch between the voltages on the fly.
Lastly, the horrible sounding intake fan that blows on the CPU.I took it out, and put a bigger fan directly on the CPU heat sink and run it at 7.5 volts,roughly half speed.(I think this fan was a poweer supply fan that I salvaged from an older Mac)Dead quiet and very effective.The intake fan will be replaced with a quieter fan sometime soon next time I'm in the mood for some tinkering.
Bottom line...this Quicksilver is less than half it's original noise level and runs great.Heat buildup is not an issue.Dont be afraid to experiment with different types of fans, dont think you have to exactly match the CFM to what was in the there.Stores and websites that sell PC parts have lots of options.