When your Mac starts up, it typically issues a very pleasant chime. This chord indicates that the Mac has passed what is called the POST test. That's a test of the built-in hardware of the machine.
If you ever hear a series of beeps on startup, your Mac is not passing POST and a hardware issue of some kind is being detected. (Once upon a time, the POST test failure sound was a car crash, a noise that scared some users so badly that they believed their computer was about to explode. Apple wisely changed this.)
Interestingly, the POST test only occurs on startup—not during a reboot. If you're having a problem with your Mac, you should shut it down then boot it back up so that POST gets a chance to run.
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