A number of people have been complaining about a fan control issue affecting their Corsair Hydro Series H100 Extreme Performance Liquid CPU Cooler. After tearing into some units, Corsair was able to root out the problem and confirm an issue exists with a "small number of H100 units with lot code 11359403." The lot code is printed on the box, and if yours matches up, Corsair recommends running a simple test.
"If you have this lot code, test your installed unit while the system is running by pressing the profile change button," Corsair says. "If you push the profile button on the top of the cooling unit and the indicator lights change, but the fan noise remains the same, we recommend contacting Corsair technical support at ramguy at corsair dot com to arrange for some replacement parts that will correct this problem."
If the lights don't change, Corsair says resetting the firmware will solve the issue in most cases. Alternately, you can contact Corsair customer service and arrange an advance replacement if you'd rather not press your luck with a unit from a problematic batch.