Noise in 340...help

Something strange happened today when I went to start my car....just to hear it run. First of all it wouldn't start..that was my bad because I knocked a wire loose when cleaning under the hood. Once I located and re-connected the wire I turned the key and immediately it started BUT I heard a noise at the same time that it "fired". It wasn't loud but obviously different. Now while it runs I hear a "knock". The noise does not go away. WHY!!! What should I look for? It still runs smooth but the knock is evident. Any advice? FYI...it is cold here. Well only about 30 degrees.