could be the wind but i would describe that more as clicking or rattling not scrunching. Does the sound change with the wind? If yes, thats your answer.
My first thought was mice. If this is an all over problem then not likely. If its localize to one or two areas and moves around then yeah could be.
Oh, i have also heard that when its really quiet you can hear termites chewing. Its easy to pull up an edge of vinyl siding to take a peek. That may not tell u much tho.
Good luck and let us know if you pin-point it. I would LOVE a recording of the sound to use in my online home improvement radio show. Contact me at my site if you are interested.
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