Different people have different noise threshold, and different ability to filter out noise. I couldn't sleep next to a running computer even using large low-speed fans until I built one using a Reserator. I have friends who can sleep next to what are essentially heli turbines. The big annoyance of the Reserator is it's a pain in the ass to carry around.

+1 for the Noctua fans and coolers too. When I need a full tower I always swap out the stock fans and coolers for Noctua.