sometimes, when two metals or a spoon and a plate are vigourously rubbed together, why do we get a feeling as if we are about to burst???
Total WAG, but some friends and I had a similar discussion about this phenominon in regards to fingernails and blackboards. Our conclusion: it’s an activity that produces high frequency sonic vibrations which irritate our nerves and may possibly trigger involuntary neural firings (there actually are proven ways of triggering irregular neural activity with patterned electromagnetic fields).
Makes sense that since low frequency harmonics produce soothing sensations there might be an opposing effect.
http://www.straightdope.com/classics/a2_317b.html
Cecil discusses the nails-on-chalkboard phenomenon. The total WAG some researchers came up with: the 3-6kHz tones produced by such annoying noises are similar in frequency to macaque warning shrieks. So it could be a hardwired monkey reflex to become aware of danger.
But that theory is totally unsupported. Your WAG is as good as theirs.