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Let's see, 49CFM @ 20.1dBA for the S-Flex SFF21E, 33.5CFM @ 8.7dBA for the S-FLEX SFF21D, 56.3CFM @ 24.5dBA for the Arctic Cooling AF12...
All three of those fans seem rated rather optimistically, but all should be pretty low noise. I suppose the question is, how...