Asteroids travel through space making a noise like a powerful but subdued engine.
Asteroids are usually locked into orbits, but if a comet comes by, they can be bumped out of their rut and become dangerously unstable. It's only the fact that everything is locked into an orbit which prevents collisions in our solar system. Any asteroid that gets loose is certain to crash into Earth within a matter of hours. It's just barely possible to evacuate Kansas City to a distance of 100 miles in 48 hours. This requires lots of airplanes.