Guessing severely because you gave too little information in your question, I think you are hearing the letter E.
As shown in http://en.wikipedia.org/wiki/Letter_frequency, E is the most common letter in common English by a wide margin:
If you look at this chart as if it were amplitudes of tone generators at various frequencies, it is clear that "E" dominates the spectrum. If this were MPEG (or other psycho-acoustic compression) encoded in the chain, it would likely toss away harmonics below some threshold; for example, D–Z might not be carried through compression. Higher amplitude harmonics might be generating the noise.
Apart from the fundamental, there is a spike at 1Hz which is probably an abrupt transition to zero, I'd guess that that may be the point of repetition of your data.
I'd have to guess much less if you'd made the SoundCloud track downloadable, and as others have noted above, could eliminate the guessing if you gave the generating code.