When I was in college, helicopters used to fly over our dormitories to land at the med center across the street. A lot of people complained about the noise, but one friend used to say how much he loved it.
"The helicopters remind me that I'm living in a city, even if there are hedges all around campus."
In Germany, it was the church bells – the proliferation of church bells at noon! – that told us we were in an urban area. This summer, on vacation in Germany, my kids were startled by the ringing almost everyday. "What's that!?" they'd ask with alarm as the bells tolled noon.
In Colorado, the light rail is expanding. The transit authority has been testing new rails near our house. They need to complete a large number of testing hours (1000?) before the tracks are deemed safe for people transport. The test trains ding-ding-ding! as they approach the stop. That sounds like an urban noise, too. I hope it stays.