Really loud engine knock

Hi there!

It is beginning to get colder here in Norway, and with the cold, comes a scary sound from my engine. It sounds almost like someone bangs inside the motor with a hammer, and it runs rough with a valve-ticking noise. After a little while, the banging dies down to a hard knock every other 5 seconds or so, and the ticking noise goes away. This goes on for about 2 minutes, and the the enginge purrs like a kitten. Any ideas? Is my car dying on me? I do have a crack in the flexplate, but that shouldn't make the engine run rough?? The ticking noise could be the exhaust-manifold. I suspect a crack there somewhere, but who is the guy with the hammer?
My car is a -93 Ltd. 4l HO with 180k (kilometers) on the clock.

perhaps your engine oil is thickening up more in the colder months and making it harder for the engine to circulate on the colder mornings?? that would be why it goes away after a couple mins...the engine is getting warmer which will thin out the oil and thus easier to circulate. dont know what weight of oil your using, but maybe try a little lighter weight. hope that helps..