Rattling and tapping in the front of the engine on the 2002-2016 Nissan Altima V6 is very evident, especially on cold starts. This rattling noise should not be taken lightly as it warns of upcoming timing chain failure. 

The cause of the noise is faulty timing chain guide rails and timing chain tensioners that wear out well ahead of schedule. These components are there to keep the timing chain tight and properly positioned. When these parts fail, the timing chain becomes loose, and pieces of the timing chain guides break off, falling into the oil pan. 

To remedy the situation, Nissan/Infiniti has made revised guides and tensioners, but replacement can be very costly. 

Proper oil change intervals and oil type may help slow or prevent the situation.