Why do I hear low, metallic twanging as trains arrive at the subway platform?

I mean the MTA subway, in New York. If you’re standing by the very end of the tunnel where the train is going to emerge from, it’s really noticeable – dozens of low-tone twangs, overlapping. You can hear them echoing even before you can see the train’s lights, sometimes. What is this sound? If I had to guess, I’d say it might be something to do with the way it moved on the rails, but everything I picture (like trains juddering ever so slightly in and out of contact with the rails) seems implausible. Any thoughts?