The cause of clicking -- and why it increases at slower RPM, accoding to JMC:
In traditional [DC] fans, the most dominant acoustic noise is due to the fan's motor switching noise. The stator motion is a square wave that is switched on and off before and after the peak torque position. This motion causes a small amount of undulation in motor torque, producing an audible noise caused by the lower frequency commutation operation. Each small torque causes a minute contracting of the entire fan structure and results in an audible clicking noise while the fan is operating.
Iam guessing that the stator motion has to operate at some multiple of fan RPM. Take 2X. this means for a 2000 rpm fan the clicking would be at 4000 Hz -- and likely masked by turbulence and blade edge whine.
Slow the fan to 800 rpm, and now the clicking is smack dab in the midrange (1.6 KHz) and there is less noise to mask it. Plus each noise can be heard more easily as a discrete click.
Perhaps with the Papst, there 's some particular component in the circuit that is prone to -- whatever qualitiy makes the clicking more prominent.
Just hypothesis until some fan engineers I have been begging for info tell me something.