The engine problem sounds a lot like a common problem area in many GM cars, I don't know if it happens in Mitsus though. What happens is the sides of the pistons, be it the piston rings or the sides of the skirts, wear a little bit over time against the cylinder walls. This creates a gap between the sides of the cylinder and the piston. The piston then bangs around a little bit in the cylinder, making the engine sound a lot like a diesel. The reason this goes away is because as the car warms up the metal in the engine expands, making the piston larger and the cylinder smaller, thus elimintating the gap between the cylinder and piston causing the banging.
It's reffered to as piston slap. In GMs it is often considered a good thing. When the engine is warm the piston fits in the cylinder perfectly, meaning that there is little friction. This causes an increase in gas MPG as well as performance. Compare this to a car without piston slap, where the piston and cylinder fit perfectly, and when the engine warms up they are too tight and create more friction.
I had this problem with my car, and everybody said to leave it be. Usually it takes a long time to get worse, and is a VERY expensive fix (you need to tear down the engine). My car however got really bad. In talking to people, GM almost never fixed this problem, and claimed that it was normal in these engines. Once again, my car was so bad that after talking with GM my service rep he was able to get me a brand new (longblock) engine. Thats how much they didn't want to deal with it, they gave me a $6000 (part+labor) engine for my $50 deductable.
If the car is under warranty, try to get it fixed now. If they tell you that it's normal for the engine to do that, tell them that they need to fix all the cars that don't sound like diesels, because then that itsn't normal. This is all assuming that piston slap is indeed your problem, but it sounds to me as this is most likely the case, between the symptoms and your dealer saying it's normal. On the other side, cars with piston slap have been known to go 200,000+ miles without it getting any worse.
Submitted for the approval of the Midnight Society