I'm trying to diagnose a ratting in my engine.

I only hear it in Drive, not in Park or Reverse. It gets especially loud when the engine shifts from 1st to 2nd gear.

I've also been having an issue with an oil leak lately. I don't know if the two can be related.

 

Manual or automatic?
Location of the oil leak? A common source of oil leak is the camshaft endplug next to the EGR valve on the passenger side of the engine. Cheap and easy repair if that's the case.