It was a loud, repetitive sound.
Is the comma optional? If not, what is the difference in meaning between them?
With the comma, the sound was both loud and repetitive. Without the comma, it was a repetitive sound that was loud. I can see no difference, so I call that comma mandatory because without it, the reader wonders where it went.
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With the comma, the sound was both loud and repetitive. Without the comma, it was a repetitive sound that was loud. I can see no difference, so I call that comma mandatory because without it, the reader wonders where it went. Of course, you can imagine contexts where it would be justifiable to omit the comma, for instance, the passage "It was a repetitive sound. It was a loud repetitive sound.