Really depends what you do with it.
I've owned an iBook, PowerBook and two MacBook Pro's and they've all been silent during normal use. Working, coding, watching videos, playing music, surfing the web. But if I were to encode video, launch a game like WoW? They get super loud. Everyone of the Mac notebooks I've had get very loud under those load scenarios.
And that is where the complaints come from that Apple designs their notebooks for silent operation when you're not doing much but when you really push them they get very noisy.