Intel boards and systems are designed to indicate USB readiness by a series of beep codes during POST, before video becomes available. These beeps mean USB is powered and initialized.
Devices such as a pen drives or USB CD/DVD ROM drives attached to an external USB port, generates a beep once the device is recognized, powered, and initialized.
These beep codes do not signal any error. They signal USB and external device readiness during POST.
For a complete list of beep codes, see the Technical Product Specification (TPS) for your board.