On some Mac models, you may hear one of these beep sequences 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 five seconds
Your Mac isn't detecting any memory (RAM). If you've recently added or replaced memory, make sure it's installed correctly.
Three beeps, then a five-second pause, repeating
The memory in your Mac didn't pass an integrity check. If you've added or replaced memory, make sure it's installed correctly.
Three long beeps, three short beeps, then three long beeps
To resolve a firmware issue, your Mac is restoring its firmware. You may see a progress bar, after which your Mac should start up normally.