Silence isn't goal, controlling the water temperature is. Not having the fan screech is nice, but having it willing to run at <20% duty cycle is more useful.
Bill, my sample of three fans all had different ratios of turbulence noise vs. PWM noise. Total noise seem to decrease (measured with an uncalibrated ear) with reduced duty cycle, but noise vs. CFM certainly increased.
If control weren't the big issue, I'd say 25 kHz or more. And maybe pulse between 12V and 3V (not ground), so that the fan's electronics would stay powered. Projects for another day.
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