There are two types of single beeps - one continuous tone, or a one-second repeating tone.

One repeating beep means something has gone wrong in a self-test. Modern firmware groups a number of errors into one code - can be GFI self-test failure (common) or pilot signal self-test failure (far less common), or firmware/EEPROM failure (very uncommon unless you're editing firmware).

A constant, nonstop beep means that there is no ground. Ground is detected by passing a small current from an AC line to chassis ground. No ground is most common when using molded extension cords or adapters with a D-shaped ground socket, which has intermittent connectivity.