To me it literally just sounded like Spanish, but in a high pitched whistle. Some of the sounds don't reproduce 1:1 in a whistle, but the whistling is adapted to still sound similar and comprehensible. I thought I would be able to do this myself by whistling out Spanish words, but it appears that it takes some practice to make all of the correct sounds. I tried in English, Spanish and Hungarian and I'm only able to produce musical notes, not distinguishable letters.
