The higher the diameter to thickness ratio, the lower the static pressure capability, given same ratio of center hub, same blade design, etc (all else being as equal as possible). The lower the static pressure capability, the more difference there will be between a fan's free-air rating and the airflow rate as implemented in a system. The more impedance to airflow, the more difference there will be.
In other words, use the thickest fan possible unless you have a particular need for a thinner one. One such example is mentioned previously, that if you are unfamiliar with how a particular fan will respond to a large reduction in RPM, fan throttling to reduce noise, a thicker fan might have too much torque.
The best method to combat this torque is current control, rather than voltage or PWM. A simple series power resistor will result in quieter operation per RPM, than running an fan straight from 5V rail, from 7V as difference between 12V rail and 5V (ground connection), or PWM.
The significance is that a fan does not draw constant current, each time a different part of the motor is energized, there is a larger current surge and sudden torque increase. This moment is when the pulsation or click sound is caused. By reducing current this spike in current consumption is capped. What remains is merely knowing what value of resistor is necessary per a given fan and this may be counterintuitive because the lower the initial RPM (@12V) spec of the fan, the larger the resistor value needs be to effect a significant change in RPM.
For example if a fan was spec'd as 4000 RPM @ 12V, a 68Ohm resistor might be too high. If a fan was spec'd as 1800 RPM, a 68 Ohm resistor might be too low, but these are only generalizations, only with the specific fan and the system airflow needs can it be optimized the most.