So I have a couple of questions.

First - The engine is very loud, and it has been for a long while (since I bought it, 5 months ago). The valves are ticky, and there is a strange buzzing sound coming from the left of the engine bay. [In video #1] It is currently throwing a SES light for the code 'P0420'. AutoZone said it could be a catalytic converter failure or a large vacuum leak. There is also a slight knock, but I'm thinking that is a normal engine sound coming from it.

Second - The transmission has horrible delayed shifts. With shifting, say 2nd to 3rd gear, there is a delay between the shift. I know that is a normal thing to have a delay, but it has a big delay. It's almost completely letting off the gas then shifting into the next gear. It isn't smooth either. I've only driven manuals and automatic trucks, so I might not be used to this, but I don't think it's normal. The transmission fluid has been checked and it is past the line actually. Looks like it's overfilled. I'm going to be doing a fill and drain very soon, but could that be the issue? Maybe the last owner didn't put the nissan stuff in? What is the correct way of doing this?

Video #1 - Loud ticking and Buzzing:

Video #2 - Transmission Shifting:

 

I guess this is normal?

 

Odd on both of these. Have you changed the engine oil? I would bet the high tranny fluid would be the cause for the transmission issues.

 

I'm about due for an oil change but yes, I've changed my engine oil twice since I've bought it. (Every 3k miles.)

I will change my tranny fluid and drain it a little when I can get my hands on some of the matic fluid from Nissan. Hopefully that will fix it. Is overfilling on tranny fluid bad for it, as much as an engine?

 

Yes. It can cause just as many issues and it created extra pressure in the transmission.