i have asus sabertooth z77 and the assistant fan 2 speed is very slow as it is between 972- 1020rpm and the assistant fan 1 speed is 4666 rpm , the assistant fan 2 make a very noise sound , this problem happened suddenly .
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thats why i never recommended that sabertooth board. its a total gimmick
looks like you have a bad fan. email asus, explain to them, and ask for a replacement
You can take a can of compressed air and see if there is any dust buildup in the fan that would be causing it to slow down and make noise.