[TSquare]

Most likely it was the gas tank pressurization. It will run from time to time.

 

[TSquare]

There is no on board indicator and it does not run often. All I have is the list put together on this forum about various sounds from the Volt.
http://gm-volt.com/forum/showthread.php?17269-Beep-Chirp-Chime-Horn.-A-guide-to-what-your-Volt-is-saying