Maybe he sees the vacuum cleaner suck up things and is scared. It might not necessarily be the sound.
Dogs have much better hearing than us, they are sensitive to high frequencies which are emitted by vacuum cleaners!
Dogs have sensitive hearing, so something as loud as a vacuum will scare them. Dogs hear at a wider range of frequencies than humans. The low end of the range is similar, but dogs hear noises up to 45 kHz, while humans only hear sounds up to about 23 kHz. This means that they could be hearing and responding to sounds that we can't hear at all. Cats can hear sounds as high as 64 kHz, bats up to 110 kHz and porpoises up to 150 kHz! Younger people and animals generally have more acute hearing than older ones. Deafness is not uncommon in old dogs and some have hereditary deafness. At the high and low ends of the frequencies that we hear, sounds must be louder, or more intense in decibels, for us to hear them. So, a dog could hear a siren farther away than we could. Pain results from sounds that are much louder than our threshold of hearing. Dogs could feel pain from sounds that weren't painfully loud to us. Very loud sounds can hurt the ears and if a sound seems too loud to you, it is probably more so to your dog.
Because they are so loud and their hearing is more sensitive than ours. It probably hurts their ears.
Some do some don't! Mine loves to bark at it and try to attack it. I think it is mainly the noise of it, since a dog's hearing is much more sensitive then ours. Good question:)
