What is it when there's a knocking noise from motor while I'm driving?

Without more detail...

It could be...

the grade of gasoline is too poor

knock sensor may have failed

worn-out rod bearings

loose flexplate to torque converter bolts

worn-out wheel bearings

missing anti-rattle springs in the brakes

something hitting the fan blades

loose parts

block heater cord banging against car

gremlin on trunk lid wanting to get in, etc.