Quoted from PjM:
JJP should be sending you an isolator! I had the same noise and that is not acceptable. JJP sent me one. You should do the same because again, that is not acceptable to have that noise.
To be honest, I don't know how the machines went out like this because the noise is pretty bad. I'm not the original owner and the game is 1.5 years old now so I just went and spent $20 locally. I decided to test the audio channels after install and find left and right are switched even though everything is wired correctly. So I just swapped the RCA connectors on one end and it's now correct.
(If I was original owner, I would have asked). Install completed.