Oxygen Sensors Maybe?
 

So over the last few weeks my engine has developed a slight knock. It shakes a bit throughout the rpm range but its not too bad. My wife doesn't even notice it but what does she know right lol..

Anyways the cel is on and i checked the code and came up with 420 which if i remember has something to do with catalyst efficiency. Also noticed when its colder outside and I get on the gas a darker gray smoke comes from the pipes but goes away after I get back down to normal driving rpms.

Engine has 108k miles on it and I'm pretty sure they're original sensors so I know its time to change them out but my question is would bad o2 sensors cause an engine knock and smoke to come from the pipes?

A bad sensor can cause the engine to run lean, so it could be the solution. However remember it's the front sensors the affect the engine, not the rears (and running rich would trip the rear sensor).

Ok well I plan on replacing them but where should I get them from?

The stealership, autozone or advance auto? Heard somewhere to not buy shit like this from autozone....

Sensors can cause it to run lean, but it wouldn't cause a knock.

a knock is a sign of a SERIOUS problem. See a qualified mechanic and stop guessing or you'll buy a new motor.

Or very rich. A very rich or lean condition can cause misfires or even knocking. However, the sensor must be tested first to determine if it's the problem.

As for sensors, OEM Ford or bust.