While under load, the engine sounds like there is a bunch of marbles bouncing around in the engine. Revving the engine in neutral does not create the noise. Filling the gas tank with 93 octane instead of 87 "solves" the issue with the noise.
Filling up with 93 octane seems silly and expensive for such an old car. Some Notes:
Prior to the issue occurring the spark plugs were replaced while troubleshooting a misfire, that has been resolved. I double checked and the plugs are correct for the engine.
The car has the 1.8L engine and ~200k miles
Is it possible that this is an issue with spark plugs or something else?