I know that sound!

Boomer did at here too. There is a tuxie cat that comes to sit on our windowsill. Boomer would go into his "territory" mode and make this low groaning/growling sound that would wake us from sleep sometimes because it sounds so eerie. If the outside kitty would not leave or come near to where Boomer was "guarding" - usually on the back of the sofa near the window - Boomer would start to hiss and thrash against the window Dylan would be doing it with him, too.

He is very protective isn't he?