Out-of-universe, there's obviously the startling, unsettling primal instincts that Spielberg puts his fingers on with those huge horns. I don't live in the US or near a sea but I would totally understand why the horn would be likened to foghorns or tornado horns, designed to set off an internal alarm.
With that out of the way, I have an admittedly grim take on the lots of the horrifying elements throughout the movie, which was set already in the beginning, when the narrator (Morgan Freeman) began his speech.
No one would have believed in the early years of the 21st century, that our world
was being watched by intelligences greater
than our own. That as men busied themselves
about their various concerns, they observed
and studied. Like the way a man with
a microscope might scrutinize the creatures
that swarm and multiply in a drop of
water. With infinite complacency men
went to and fro about the globe, confident
of our empire over this world. Yet,
across the gulf of space, intellects,
vast and cool and unsympathetic regarded
our planet with envious eyes. And slowly
and surely, drew their plans against
emphasis mine, taken from IMSDB
I personally think this was a showcase of humanity's inexplicable weakness compared to the aliens. Aliens are indifferent to humans' reactions, like a scientist exterminating bacteria they no longer need in a lab, or exterminating vermin with insecticide in an apple garden. The horn was perhaps a signal to other aliens, that 'this tripod has started cleansing this area', or a routine sound the machine makes while its cleansing operations start to load.
This interpretation would be consistent with some other elements incorporated into Spielberg's machines, such as the 'indifferent sack' and the style people are thrown in it resembling something like a jar, if a kid was trapping butterflies in it. It's not too comfy but it's spacious enough to allow limited movement, not rigidly handcuffing people to walls or somesuch, as you would expect from some sci-fi tropes. Another example could be using blood as some form of 'fertilizer', as soon as resistance started to break down.
These themes throughout the movie lead me to believe nothing really was fine tuned in the machines as they were depicted as all-powerful, to which whatever humans did was inconsequential.