the air being pushed out/around from aircraft engines probably. most running engines produce some low freqs, and also slow-rotational things.. like cars' wheel

the air being pushed out/around from aircraft engines probably. most running engines produce some low freqs, and also slow-rotational things.. like cars' wheels thumping on streets and roughnesses there.. esp. thousands of those. And then combinations of almost same freqs produce very low differentials - something on 50hz and another on 53hz will yield some 3hz. Which cannot be heard, it's to be felt. Another similar silence happens if one is in a street/ suburb/ block-of-flats full of airconditioners-on-walls when the power goes off.