I have a 2009 24" which Apple rates at 21db for noise. The 2011 21.5" is rated at 18db. I don't know much about db ratings so this may be significant or not. What bothers me most about the 24" is the fans. The computer is on 24/7 and the constant low level wooshing of the fans is noticeable. My MBP is much quieter in comparison. So has anyone had a 24" and then switched to a 21.5" in a quiet environment? Differences?