1983 300D strange deep rattle when oil cap is removed
 

About a month ago I replaced my valve cover gasket, however I noticed while doing so that the timing chain seemed a bit loose. I didn’t do much at the time and kept driving. I now hear a light rattling noise at certain RPMs and I wanted to see if I could get a look at the chain through the oil fill cap. I took the cap off while running and I hear a deep erratic rattling, is this the chain or something worse, and can it be solved with a new tensioner spring?

Here is a video of the noise:
https://youtu.be/H2-zOs3qKIc

Very insteresting noise... in your car, the tensioner spring is only for startup before oil pressure has built. So, I doubt that will fix your issue. Perhaps it's the tensioner itself? Or a chain guide?

Vacuum pump??? Agree the spring will not fix the tensioner. If that is a clean/keeper car look into it as soon as you can to prevent major engine damage.

Does it use much oil?

No, it leaks from various places but it doesn't burn any oil from what I know

That does not sound good.
I’d be pulling that cover again asap.

Poke around with a mag light and see if anything is loose.
If your tensioner, rails or anything else chain related have gone off you’ll find your slack in the chain. If not, Sugar Bears idea re: vacuum pump is a good one.
I’d be pulling that off next.