I don't believe that is what encryption actually sounds like. Rather, it is the sound that your scanner/receiver makes when it determines the signal it receives is encrypted. (For example "Beep Beep Beep" from your smoke detector isn't the sound of smoke, its the sound made when smoke is detected.)
While the newer models of scanners have "things" that either skip or give busy signals when encryption is detected, older models will receive and "hear" the actual encrypted signal. What kind of sound it makes depends on the type of encryption used such as ADP (an awful racket that makes you want to turn the scanner off) or AES/DES which sounds like a loud hiss.