What do the beeps during the Power-On Self-Test (POST) mean?
This test runs whenever the computer is powered on after being fully shut down (the power-on self-test does not run if the computer is only restarted).
When a fault is detected during the Power-On Self-Test, you will not hear the normal startup chime. Instead, the system will beep as explained below. Macintosh Computers introduced after October, 1999 use a revised set of power on self-test beeps during startup.
1 beep = no RAM installed
2 beeps = incompatible RAM types
3 beeps = no good banks
4 beeps = no good boot images in the boot ROM (and/or bad sys config block)
5 beeps = processor is not usable
In addition to the beeps, on some computers the power LED will flash a corresponding number of times plus one. The LED will repeat the sequence after approximately a 5-second pause. The tones are only played once.
Note: In this case, a flash is considered to be 1/4 second or 250 ms or greater in length.
The information in this article came from Power On Self-Test Beep Definition – Part 2!