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Originally posted by internos
My first impression is that it sounds like an engine which is vibrating in partial or full contact with some metallic parts (i.e. autoclave, refrigerator engine).
I got one and it sounds similar (not so much but it sound similar).
Is there any other quiet moment during the daytime (i.e lunchtime) that you can try to hear it again?
Is there any way to ask to someone what's the origin of this noise (i.e. owner of the shelter?)
and one of them likes to sleep in that shelter when it's rainning