Go on then what is a wokka?
A Chinook
How could you feel them if you couldn't see them?
You've obviously never lived / worked / visited somewhere that happens to be under their occasional / frequent flightpath.
Very distinctive sound and associated vibrations which seem to penetrate everywhere.....
Take a look at the YouTube videos, obviously nothing like experiencing the real thing, but you might get an idea about the sound pattern and why they are called wokkas.