Why do I hear beatings with stereo headphones?
Short answer
Dependent on the auditory stimuli used, it sounds like binaural beats to me.

Background
Binaural beats are the resultant perception evoked by the interaction of two auditory tones with different frequencies when presented to both ears (Lane et al., 1998). The presented tones should be below 1000 to 1500 Hz and differ in frequency between 1 and 30 Hz. For example, if a pure tone of 400 Hz is presented to one ear and a 410 Hz tone in the other, an amplitude modulated standing wave with a frequency equal to the frequency difference (10 Hz in this case) is perceived. Note that these 1-30 Hz beat frequencies are below the dynamic range of acoustic hearing. Hence, the beatings are the result of neurophysiological interactions in the brain, and not the result of physical interactions of the auditory stimuli impinging on the inner ear. From an experimental point of view, limiting the sound levels to 60 to 70 dB should prevent gross cross-over stimulation to the other ear.  

Binaural beats are thought to be elicited in the auditory brainstem, and specifically in the the superior olivary nuclei, when the two wave forms mesh in and out of phase within the brainstem.

Reference
- Lane et al., Physiol Behav (1998); 63(2): 249–52

Source
- Web US 

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