Here is a T-128 near the Calvert Cliffs Nuclear Power Plant sounding for its semi-annual 3 minute test. Sounds like this particular siren may have run off of its DC battery backup supply for this test; a different way to test considering this is a T-128AC/DC, but it does makes sure the DC backup supply works.
Calvert Cliffs Nuclear tests on the 1st Monday of every month at noon; sounding for 3 minutes in June & December, and is growled all other months. Enjoy!
https://www.youtube.com/watch?v=Q5OIIhhgsG8