Most people find this sound unpleasant, the usual explanation is "because you associate it with something unpleasant - having your teeth drilled".
I think it is more fundamental than that. Humans, and presumably most mammals, have an evolutionary and instinctive dislike of mosquitoes and midges. When you hear one, it really puts you on edge; you can't relax until you've killed it or it's gone away. It's hardly surprising that a dentist's drill has the effect.
The same goes for garden strimmers. Lawnmover, power drills etc are annoying simply because they are loud and monotonous. But they don't annoy me half as much as the sound of a much higher pitched strimmer.
Debate Magazine
