Strange sounds on the bus

So this isn't a personal story, but I thought it was funny. A friend of mine was riding with an American (who didn't speak any Portuguese) on one of the public buses. Every time they stopped, there was a small explosion of hissing. The American asked, "Why is it that every time the bus stops we hear all this 'sss...sss...sss'?"
I laughed when I found out that it was a sound that I hear all the time but don't pay that much attention to: people saying "Com licença" (which means "excuse me") when they were making their way to the back of the bus to get off. Brazilans don't say very many full words; they tend to cut a lot of letters off, which makes listening a chore some days. What this American was hearing was "cença" (pronounced sen-suh), but her ears weren't picking up the vowels, just the s-sounds, much like what we hear when someone is whispering.