A 2001 Pathfinder has OBDII or III. The oxygen sensors should last longer than 75,000 kilometers as the new ones are either heated, or have coatings that can handle the exhaust gas contaminents better. 75,000 kms was the maximum for the old type single wire unheated O2's. Reduced fuel consumption is usually the biggest indicator that your O2's are going. From 1995 onwards, most cars started getting two O2 sensors, one before the cat, and one after. If you're lucky, you many only need to change one.
Although O2 sensors are a common reason, try not to jump to that conclusion. The best thing you can do is to pull the codes from the computer. On Chryslers, you can do this by turning the ignition key a set number of times without cranking the engine. On GM's, you jump the A and B terminals in the Assembly Line Diagnostic Link connector. Every manufacturer is different. The Pathfinder has a fair following, so I would find a message board much like this one to find out how to pull codes.
Checking the Manifold Air Pressure sensor can only be done by either pulling codes, or using a diagnostic computer. I'd be a little surprised if it used a MAP sensor as most newer vehicles use a MAS which is more accurate. MAP's tend to be more dependable though. MAF and MAS problems usually show up as rough accelleration as the sensors cannot properly meter the density or airflow. A simple test for a MAS is to lightly tap it with the end of a screwdriver handle to see if the vehicle stumbles. I wouldn't recommend it in this case as I have my doubts it is this problem.
As papasmerf mentioned earlier. Even an air filter can throw a code if it is too restrictive due to dirt or buying the wrong type with a crappy filtering medium. The new computers are very sensitive and even leaving your gas cap loose can trigger a code.
In short, pull codes before jumping to any conclusions and monitor your fuel consumption.
As for the rattle, they are hard to chase because they resonate through the entire system. I would be more inclined to look for a heat shield than a worn gasket. Heat shields are usually just tack welded or riveted on and rust easily due to temperature extremes and exposure to salt and water. Again, try and find a Pathfinder forum. Sometimes these things are a systemic problem. Lots of room under there Escohort. You should crawl under there and take a look.
