Thanks. That may be another face of the QC issues that others had with this model (I recall reading about clicking). I might give the S-Flex another go with the F model because it starts reliably at 5V. I originally chose the S-Flex over the Slipstream for longevity because I am still not sure what to make of the sleeve bearing and relatively short life expectancy of the Slipstream. The last thing I'd want is the fan stalling 3 or 4 years from now because the bearings were worn out, but then again people have been running them with no issues so far.
I have a noisy S-Flex "F".
I recommend the Global Win NCB fans. I have three of those and all are excellent. All three cost less than the one S-Flex. They are only ~1300 rpm, so the "F" does gives better cooling at 12V.