NeoSeeker have a review of Thermaltake's Silent Boost, a very good cooler in both the temperature and the noise level categories.

The fan is indeed very quiet compared to many other heatsink fans. However, the provided rating of 21 dB is not very accurate, as the fan is audible over a whisper power supply fan. However, out of the heatsinks listed, the Silent Boost is the quietest of the bunch

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