It depends on the knock. Check these-
-a big nail in a tyre
-A loose or worn bearing. If it has cup-and-cone bearings, they need 33 ft-lbs of torque to preload them right- check the specs.
-there may be something in the brake drum.
If it increases with road speed, and out of round or out of balance wheel will cause a shimmy in the steering as above, see Jerry's comment above.
