Odd engine noise.

This morning I started the car. (It was only 5 degress (F)) and it turned over slow as usual in cold weather but started. After it ran for 20 seconds, it began making this slapping, clattering, chattering, knocking noise. It sounded almost like hitting a marble with another marble over and over at a very fast rate. The noise was going at the same speed as my engine. The noise was almost as loud as the engine itself. The noise went away after a minute. I've never noticed this noise before even on days when I have managed to get the car to start on days where the temperature is -3 F. (Although that's only a few days out of a year.) The only difference between then and now is I'm using thinner oil. I'm assuming it was just because of the cold but it might be signs of something happening. I know it's harder to tell what it is without listening to it, but do you have any ideas of what this marble hitting a marble noise is? Thanks.