My (limited) understanding is that it's the CPU. But right now my MacBook Air is whirring loudly even though only Chrome, iWeb and Mail is open. The CPU is at 84 degrees (is that normal?) even though usage is 2% User, 1% System, 97% Idle. Activity Monitor doesn't appear to show anything unusual. So how can I find out why my Mac is so hot and battery life suddenly so bad?