2001 nissan maxima engine sound?

The sound started going off when your cooling system kicked on I would start there might be as simple as your electric fan is tapping something as it cycles.

what weight oil did you use motor sounds as if it has an exhaust leak and valve tapping sounds the audio quality is not all that great. High Mileage motor oil and 10wtx30wt or even thicker 20wtx50wt might be thick enough to help lubricate metal parts. Does it sound loud enough to hear the sound inside while driving? Mechanics don't like to tear into high mileage engines. A good used low mile replacement motor is a better alternative to patching up a old engine. Good Lick

visit an auto parts store. they will tell you if something is wrong.