A knob is a hill. Most likely open (unforested) or covered
with sheets of rock. I'm pretty sure that New Hampshire
has some peaks with Knob as part of the name. With
my corrupted system searches are a pain.
Eliot may have used the word knob instead of hill to bring
us back to the house image (door knob). I think though
that he wanted the image of a sphere (as with a door
knob) to represent the earth.
Under a windy knob / In a world of life