Imagine a flock of birds scattered about in a half acre area. Some are on the ground walking around, some are perched in trees and bushes, others are flying from one place to another. When all is well they are active. They sing and chirp and chatter. If a hawk appears overhead the birds go silent and still. Sound signals safe. Quiet means danger. Therefore, perhaps to a bird in a cage inside a house a vacuum cleaner is a reassuring safe sound.
A dog, on the other hand, is a predator. He's a hunter. He works stealthfully. Picture a pack of dogs pursuing prey. They follow quietly, waiting for a chance to leap in for the kill. If suddenly a roaring snarling beast were to appear, charging at them, would they not flee in terror? This is, I believe, why the vacuum cleaner frightens so many dogs.
So much for heady theories.
And what of onomatopoeia, you ask? Such a fun word. Onomatopoeia means a word that means a sound, the word is formed by that sound; imitates the sound. For our purposes, we have tweet, woof, and roar.
Echolalia is the process whereby a human baby learns to speak by parroting, or echoing, the words and sounds he hears other human beings making. This may be how birds learn to sing. After all, the canary breeder doesn't leave the young male birds alone in a room with a recording of Mario Lanza singing. The birds are together in the room with other birds. They can hear each other singing and are encouraged to sing along. Echolalia is key to helping a creature learn the language of his fellow creatures, not creatures of a different species. Or maybe not.
Which leads us to another Brain Teaser. What language speaks the Vacuum?