depends
It depends on your energy saver settings and what you're doing on it.
If you're doing anything processor intensive while it's set to automatic (or esp. highest), the fans can get sorta noisy. If you run say distributed.net rc5 client on it and set it to highest, within a minute, the CPU fans will be running at full (very loud). Full hurricane mode is **VERY** loud (if your machine hangs, if you hit a kernel panic, or if you run something that doesn't provide fan control).
While idle and no load, it's pretty darn quiet.