The effects of a consistently disappearing and reappearing spontaneous otoacoustic emission (SOAE) at around 2280 Hz on microstructure for pure tones of varying durations in a 33 year-old woman with normal hearing was studied. The SOAE began to appear after 10–15 min in a quiet test room and increased in level by up to 22 dB over a 30–40-min period. The SOAE was measured every 12 to 15 min. Between measurements, the subject performed a signal detection task for pure tones with total durations varying from 20 to 320 ms. The signal frequencies were within a ±30-Hz range relative to the SOAE frequency. For signal durations of 40–320 ms, there was a local dip at the target SOAE frequency when it was either not detectable or its level was lower than −14 dB SPL. Subjective threshold levels were as much as 12 dB better than those obtained when the SOAE was −6 dB SPL or greater. The results suggest that a region of the cochlea with high sensitivity and instability can be put into self-oscillation producing an SOAE, possibly by a change of efferent activity. Hearing threshold is affected possibly due to adaptation or masking.