Engine making a knocking sound

"knocking" is usually used to describe the sound from a bad crank or rod bearing. "clicking" is used to describe a valve adjustment problem.
Adjusting valves is cheap and easy, so it is worth a first guess. Try that first is it doesn't solve the problem you might be looking at a lot more work soon.

Going by the fact that you were driving on the freeway while low on oil, I'd say you've got a rod bearing going bad. If it's a loud knock, almost diesel sounding, the bearing is almost certainly non-existent, and you have crank journal damage. I had an '86 Toyota Corolla that did this very thing, and required a complete bottom end rebuild for it to be right again. However, if the knock isn't that loud, but definitely audible over the engine's idle, get it fixed as soon as possible, and don't drive it until then. Keep in mind that this is only a guess based on what you've described, but valves don't just come out of adjustment in a single trip.

The "knocking or clicking" sound is non-existent when the car is idle. The sound is only audible while driving.

A bad rod bearing will "rap" when you snap the throttle open at 1500 to 3000 RPM repeatedly. If you hear it, start pulling plug wires one at a time and repeat the test. If the noise goes away when one certain wire is removed, you got a bad rod bearing on that hole for SURE.

A spun or scorn rod bearing will come more apparent under load. Use Steves method to narrow it down. Good luck.