Fans are used because they are a safe and easy method of reducing heat, which can shorten the life of a computer. The noise of a computer is emitted as a combination of turbulent airflow and vibration from the fans and the hard disk. The particles of air move at a high frequency and cause pressure, when they reach the trader's ears. The ears regard these particles as noise. Noise can cause stress in a trader and cause him to lose his concentration and lose money trading.
Some traders are more apprehensive of sounds than others.
This depends on their psychological attitude toward sound. Apple computer has realized that sound is a problem and has come out with their PowerMac G4 Cube. It has no fan and therefore no fan noise. It has a targeted cooling system that eliminates the need for noisy chassis fans. Both of these computers have limited ability to use multiple port video cards for multiple high-resolution monitors required by many traders.