I believe
there is some indigenous people somewhere which has a
language whose sounds are all [] whistling; [...]
Maybe you had [Silbo Gomero] in
mind:
Maybe. [...]
I think you are thinking about the Pirah?. An
indigenous tribe in
the Amazon.
After reading the Wikipedia page, I think what I was remembering
(fuzzily) is more likely to be Silbo Gomero. The Pirah? are
interesting as well, but they match the memory less well.
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