Too cute. I agree, the sound of the squeak is setting off his howling instinct, like some dogs howl to sirens in the distance.
It seems to be related to the exact pitch that triggers the howling.
I have a dog that will howl at me when I howl at him first. He will stop if he doesn't like my pitch, or he will howl more wildly and loudly if use a pitch he really likes
The other dogs just stare at me like i'm nuts.