On some Mac models, you might hear one of these sequences of beeps at the beginning of the startup process, while the screen is still blank. These are not the same as the startup sound (chime) that a Mac can play when starting up normally.
One beep every 5 seconds
Your Mac isn't detecting any memory (RAM). If you recently added or replaced memory, make sure that it's properly installed.
Three beeps, then a 5-second pause, repeating
The memory in your Mac didn't pass an integrity check. If you added or replaced memory, make sure that it's properly installed.
Three long beeps, three short, then three long
To resolve a firmware issue, your Mac is restoring its firmware. You might see a progress bar, after which your Mac should start up normally.