When they reboot at 3AM, the fan comes on high and it is quite loud for a few seconds. In normal operation, I can't hear mine at all. My daughter uses hers as an alarm clock by changing the reboot time.
Great tip! I think alot people don't realize how much dirt accumulates inside electronics, especially ones that utilitize fans, causing overheating problems and eventually a shorted life span.
Our 612 is in the bedroom. My wife was complaining that it was waking her up in the middle of the night so I changed the update time to 9 AM. No more problems, and it's dead silent the rest of the time. If you're hearing the fan a lot of the time I'd sure check for restricted airflow and or dust buildup.