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Kerrymac - I could hear this weird two note kind of wavering. A bit like a very quiet siren. Except it didn't have a regular rhythm, it was just like a fluctuation between the two notes.
Groingrinder - The wavering sounds like there is some Beat Frequency Oscillation going on. When two sounds are almost the same frequency there is an oscillation that is perceived in the ears. You can experiment with a guitar or other stringed instrument. Some guitarists count the oscillations in order to better tune their instruments.