ExtremeTech has a guide on swapping the Xbox 360 fan with a more silent model:
Jason Cross attempted to silence, or at least quiet down, an Xbox 360 some time ago. Since then, ready-made mods have appeared to help with noise. We're talking about the Talismoon Whisper Fan for the 360, made to spec to replace the 360's stock, noisy dual-fan unit.
Getting into an Xbox 360 isn't easy unless you have the proper tools. Thankfully, a company created unlock kits (for this article, we used the kit known as V2) that perfectly fit the various tabs and screws you have to manipulate to get the 360's case open.