Most PWM fan exhibit some
level of clicking and some even whines at different duty cycles.http://www.youtube.com/watch?v=41US0XZg-GI&t=1m21s
(Example of whining noise)
Alot of the time it doesn't matter much because the fan is housed inside a case that dampens the noise to a point the user can't hear it. However, some people will build their computer to be a silent machine and/or work in very quiet environment. The noise produced by PWM fans will be much more noticeable.
I am very picky with noise and clicking/whining annoy me immensely. I've tried numerous PWM fan. Some of them click loudly. Majority of them click quietly or mildly and a few will exhibit high pitch whines.
The only fan that I have heard "ZERO" clicking or whining noise are Noctua PWM fans
. They are dead silent. Seems like noctua has their own special pwm circuit that specialize in eliminating those clicks/ticks. http://www.vortez.net/articles_pages/120mm_and_140mm_fan_comparison,22.html
This review goes more in-depth about it and also claim the noctua NF-F12 has zero audible clicking noise in comparison to the competition.