This is taken from (and an answer to) mxsmanic's "trip on métro line 9 in paris, france", at 2:20 exactly: metro on rails through the tunnel, which resonance creates this sound (very familiar to me as i live here). Interesting to notice that the sound's frequency stays around 483hz, which was (in the ancient solfeggio frequency science attributed to pythagoras) the frequency of "opening consciousness".
Author: Jsahab