Which signal converter? I need a new one.

SouthArk370Z

Before investing in a new converter, I'd try grounding different parts of the installed one and see if the noise disappears. I keep a piece of 16ga zip cord (lots of strands so very flexible; enough ampacity for most auto purposes) with alligator clips on both ends in my toolbox - comes in handy for jobs like this.

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RonRizz

Grounding is your issue here. seek and you shall find. If all else fails, I've always had good luck with audio control products.

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