Called otoacoustic emissions (or OAEs), these sounds are actually generated from inside the inner ear. They can occur either as an echo generated from hair-cells vibrating from sound, or spontaneously even in the absence of sound. An article from The Scientist goes into some amazing detail.
“In addition to amplifying hair-cell activity, these active mechanisms manifest as spontaneous movements of the hearing organ, oscillating even in the absence of sound stimuli. Such spontaneous movements actually produce sound that is emitted through the middle ear to the outside world and can be measured in the ear canal.”
It turns out, spontaneous OAEs can very between people as well. A study by Baylor College of Medecine showed that out of a pool of 36 female and 40 male subjects, spontaneous OAEs were found to be more prevalent in female subjects and there was a tendency for the male subjects to have fewer emissions in their left ears.”
Detecting these emissions helps as part of the newborn hearing test, since babies don’t always cooperate. People with hearing loss greater than 25–30 decibels (dB) do not produce OAEs at all.