Why is your 1990 Nissan 300zx knocking?
There are several different reasons that an engine will knock. Unfortunately, the most common cause for an engine knock, is when the connecting rods and the pistons have more play than they should, causing them to move around in the cylinders. I would recommend taking it to a mechanic and having them listen to it and get their opinion. Try not to drive it too much before getting it checked out, as this can cause your engine severe damage and possibly leave you stranded on the side of the road.