If that sound is, "Oooh, yeah," pick a new stranger, and try again.
See, according to modern science, there's a single word shared by pretty much every language on the planet. It's not a big word. Some have questioned if it is even a proper word at all, but it's one you've uttered in the last week. The word in question is "huh?" -- the universal sound of disbelief and/or confusion; it's the single-syllable way to say, "Can you repeat that?"
A Netherlands-based team did a study about this, analyzing recordings of conversations in 10 unrelated languages like Spanish, Chinese, and Icelandic, and found that not only did all of them have a short word that begs clarification, they all sounded alike as well. Even the Russian language had a version that was close to it, despite the fact that their language doesn't have an "h" sound (making their "huh?" sound closer to "ah?," which is still pretty darn close).
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A large part of Russian homophobia is just fear of what they can't pronounce.
So why would so many completely unrelated languages settle on the same little interjection any time the listener wants information to be repeated? One of the linguistics researchers responsible for the study, Mark Dingemanse, seems to think it's due to convergent evolution -- the process by which multiple subjects accidentally evolve in similar ways.