[richtazz]

The knock you are hearing is one of two things, both very common in high mileage 3.1's. The first is piston slap, caused by the relatively short piston skirt/high piston pin placement in these engines when they upped the displacement from 2.8 to 3.1 liters. The second cause is lifter noise. These engines use a "zero-lash" set up where the rocker arms are torqued to a specific value, rather than lashed. This arrangement doesn't allow for increased clearances when parts wear and will cause a clatter/knock at start up until the lifters "pump up" with oil pressure. Neither is anything to worry about, as long as it doesn't continue after the engine is warm, then you may have a more serious problem. Keep an eye on your coolant too, as these engines are famous for intake gasket failures that lead to coolant consumption and oil contamination.