2000 Malibu Sounds like a diesel on Startup (cold)

I'm new here, but would love some input. I just bought my first American Made car, a 2000 Chevy Malibu STD with 130K. When the engine is cold (outside air is warm), and I start it up, it sounds like a diesel for about 3 minutes, and a knocking in the motor (the 3.1) persists for about 5 minutes. Once the Engine Temp reaches 150, the engine runs smoothly. Its a really peppy engine out on the road & it runs great. But what the heck is that noise?? I just changed the oil & filter & replaced 1 qt with some Lucas Oil additive to see if that would help, but it hasn't seemed to lessen the noise.

Any help would be appreciated.

-AIM

it's piston slap. very common in that series of engines. only solution is new pistons. or just let the car warm up before driving it hard and deal with the knock. for some reason, the piston skirts bow in and causing the piston to slap around in the cylinder. when they warm up, they expand and straighten out and the piston guides smooth in the cylinder.

F.Y.I Lucas additives are not as helpful as you might think. They dilute the antifoaming agent in oil and it gets frothy. Frothy oil doesnt lube as good.just letting ya know