Why are xbox 360 so loud?

If you mean the loudness of the Xbox itself and not the sound coming from the speaker: The disc spins at twelve times the speed of a normal DVD. That means 150-200 revolutions per *second* (or almost 10000 rpm). As the discs are sometimes slightly bent due to manufacturing or careless pulling out of the case the discs start to vibrate, causing the noise.

If you play a Demo or arcade game from the harddisc, the Xbox 360 is relatively silent.