Sorry to pull up this older post, but I have a bit of experience with this kind of thing. I have done ambient nature recordings in country areas at night, to hear crickets and night birds and the like. I have used the Sharp DR480 and the SP-BMC-12's, which make for a very good combination. Going to a better set of mics definitely helps due to the lower mic-noise levels. I am also considering a good preamp and even better mics. Even so, I have made recordings of nature in Mono with a lapel microphone, and the narrower frequency response of the mic meant that the bird and insect sounds were more distinct, clearly audible in the relative silence of the recording. I did those with the MZ-R55 with the mic level on High. I must say that you're a bit brave to be leaving your gear in the middle of the forest overnight! Funnily, I, too, can hear the flanging. It is clearer in the second half of the track, sounding like a regular 'rasa rasa rasa' noise.