It could be low oil pressure or worn main bearings, but it more likley you
are experiencing "piston slap." This is a noise generated by the pistons
rocking in the bores. As the piston reach the top or bottom of a stroke, the
rod angle shifts from one side to another, changing the side trust on the
piston from one side to the other. This casues the piston to rock in the
bore. If there is excessive clearance, there is a lot of movement and the
piston builds up enough speed to hit hard enough to generate an audible
click. As the engine warms up, the piston expands faster than the bores and
the clearance is reduced, so the noise goes away. Changing oil brands /
viscosity might reduce or eliminate the noise. If not, you would need to
rebuild the engine to compeltely eliminate the noise. Since your car is
older, it is likely that the piston slap (if it is piston slap) is related
to wear. In recent years piston slap has been a common complaint even for
new engines (Ford, GM, Toyota, Nissan have all had problems). If the noise
goes away relatively quickly, I would not worry about the noise. Fixing it
might cost more that the cost of the vehicle. As long as the engine is
running OK and not using oil, just keep driving. However, it is not a bad
idea to have someone check the oil pressure with a mechanical gauge. This
would verify that your problem is not related to low oil pressure.

Ed

   Well I understand the 4.6s are (were) known for that problem that Ed
mentioned, but it kind of sounds like a bad lifter to me.