Totally normal, Don't worry. Its a diagnostic of the O/2 sensors run by the PCM. The PCM makes a squel/beep noise when this test is being run. This happens when you turn the key off.
The PCM, may or may not make the noise, but it considered normal and not a problem to worry about. Test runs anywhere from 10 to 20 seconds when the key is turned off.
Returned 2006 Silver SRT8 (vanished into the unknown!)
2004 Jeep Grand Cherokee Overland
2009 Jeep Patriot Rocky Mountain
1982 Kawasaki KZ1100 Spectre