Tyler, before you lose all happiness about your new to you truck you should know a few things, very little is diagnosed on these engines without codes. One can go through parts like a swap meet free-for-all guessing. Sounds and their descriptions vary so that's not a good diagnosis in itself. Does the sound go away with higher engine RPM and or is it more prominent at idle? Have you tried a fuel additive to see if that makes a difference. For example, bio-diesel runs quieter than most pump diesels. Some run 20% or more, do you know what the previous owner was doing? You might have plain old common "cackle" that was muffled by soy additive added to bio-diesel or a high dose of additives.