I am hearing knocking noise from my engine. The engine oil was less than normal on 2003 Nissan Pathfinder

This has been topped but the noise persist. What should I do?

Asked by for the 2003 Nissan Pathfinder
Define "less than normal". How much less?
Not enough?
The noise is a little above normal. The noise is coming from the left hand side of the engine when you are facing it.