Change your Pre-catalytic (exhaust manifold) converter ASAP. See: http://www.carcomplaints.com/Nissan/Altima/2002/
Altima has solid valve lifter (not hydraulic), It lasts longer but noisy (light diesel
noise).
So many Altima 2002 owners got a high oil consumption caused by Pre-cat material digested into the engine ; scoring the cylinder wall and piston rings. If your oil consumption is more than 1 quarts every 1k miles, you need new engine. All other repair are useless cause you have to change all pistons, and bore all the cylinders, new valve, new bearing, new head gasket, etc. Buy a new engine is a lot cheaper and you will have much lower mileage on the engine. Nissan transmission is stronger than Honda, Toyota, or any car makers.