Heya Tractor,
Rather than slowing down the fans and increasing the heat inside the case, you are better off just purchasing new fans.
Open the case up and try to find which fans are the ones making all the noise, then just replace them with quieter fans. I have found that power supply fans can be very loud as well. In a worst case scenario, you may even have to replace the entire power supply to get a quieter fan.
Some fans have an external speed control knob, but I haven't seen any that have gotten good reviews. But that would be convenient.
Hope that helps!
