The setting is current day Los Angeles... except for the goblin city that exists beneath it. Its chambers range from about 500-2000ft underground and has a population of several thousand. A lot of what the goblins have comes from things they steal from humans at night: radios, power tools, babies, and whatever other odds and ends they find lying around. Because of this, the kinds of sounds you hear in the goblin city will be a lot like what you hear in the human city, just deeper. While they certainly make a fair amount of noise, when I wrote this setting, I assumed that whatever noise they made would be mostly muffled by hundreds of feet of Earth and blend with whatever noise the humans were making up above.
But then I read this https://xkcd.com/2344/ this morning and realized that Los Angeles probably has some pretty dedicated seismologists keeping an eye on things. This got me wondering:
Would seismologists notice the goblin city?
Assume the humans do not generally know that there are goblins in their city at all, so they would not be actively looking for them.
Well, I'm convinced at this point that taking no special precautions would lead to the city being discovered, but if you have any ideas about how to the Goblins could take special precautions to hide, see related question: How to hide this underground city from seismologists, oil prospectors, and ground surveying projects?