According to its specs, the Radio Shack 8cm 12vdc fan (item 273-239, $17) is much quieter at 25db than the Directron fan mentioned above. I use two of these fans to cool my equipment cabinet, which has ventilation holes in the back but a solid door in the front (I prefer my audio equipment to be heard, not seen). They are VERY quiet and, being brushless, inject no hum into my system or into the audible spectrum. I have excellent hearing and I can hear the fans when there is no sound output; the majority of visitors cannot hear them at all at normal listening distance, and even a very low sound output level hides the fan noise completely. I will comment that not all of these fans are created equal; when I purchased them, I noticed immediately that one of the fans was noticeably less quiet than the other and exhcanged the noisier one.