Ever since the first Mac was sold back in 1984, Apple's computer line has emitted a beep or chime when starting up. It's kind of a way of saying "Hello" to you while letting you know that the computer is going to start going through its boot process. That startup chime is now a registered trademark according to a post on Patently Apple.
The startup chime is referred to by the US Patent and Trademark Office as a "sensory mark," and was defined on Apple's original trademark application as "a sound mark consisting of a slightly flat (by approximately 30 cents) G flat/F sharp major chord. Want to annoy your friends or family by playing that startup chime over and over, ad infinitum? You can download it from the USPTO here.