It is possible that it's some sort of auditory bias, but there's people from all over the world here (big tourist city and all), and it's almost always people form the US that I notice are loud. Even with the British, I can't normally hear them without being quite close, so it's not the language. Of course, I can't tell how many quiet Americans I've walked by without noticing because they are quiet.
That's interesting, I've had the opposite experience, so I'm guessing it's a perception issue. I'm from Buenos Aires and currently live in New York, both loud cities. I find Argentine tourists around here are especially loud. Maybe I'm tuned to pick up my own accent, or maybe tourists are louder than locals? (the excitement of the new?) Or maybe, as you point out, we just don't notice the quiet ones and assume they are locals...
