Sorry I never noticed this! More or less any source of noise that's loud enough and is coupled to the ground works. In fact, a lot of what winds up being a go

Sorry I never noticed this! More or less any source of noise that's loud enough and is coupled to the ground works. In fact, a lot of what winds up being a good source is actually things like ocean waves -- not just man-made noise. Ideally, you want a clear "spike" of noise at a lot of locations and not necessarily a moving source. However, something like a truck/train hitting bumps at different locations along the road could definitely do a nice job of that. At any rate, the basic idea is to look at cross correlations of the waveforms -- any noise that is clearly correlative at different seismic stations is used. Take all of this with a grain of salt -- I work in an adjacent field and not exactly this one.