DOGS...singing?

[Phalanx]

No, no youtube links here.
I've never seen "little" dogs do this, I guess they may...I've largely had shepherds and larger "mutts" throughout my life.
I don't mean that screaming howl, but this kind of sound they make, like they're making a long, singing note? They sometimes try to match the pitch/tone of a sound, eg, my old german shepherd/cattledog cross would (I don't know how or why this started) would follow a whistle with it.
But seriously, it's like they're really getting into belting out a tune, their eyes all closed 'n shit?
Does anyone know what I'm talking about here?
It's kind of, even, a "nice" sound?
Why?
Is it...that they like to sing?

My first theory is that MAYBE they try to match the pitch to cancel out a sound that's bothering them?
But then, my dog would do it after I whistled...hmmm...maybe not then
Could be why they howl though?

Ideas?

[bloody_ribcut]

maybe they hear a siren you cant.

[Psycom5k]

Maybe they are just fucking with you.

[Doc Faustus]

My dog used to sing like a humpback in the middle of the night. Then, when I caught him doing it, he gave me a "what are you lookin' at?" look.

[Phalanx]

My dog, it was like he was yknow, doing one long note, then sometimes one or two more, always a different tone from the first one - kinda like someone warming up their vocals?
I mean, he seemed to like doing it...