Install an electrolytic capacitor across the output terminals of your converter.
As big as a salt shaker. Two #10 screw terminals on top. 10,000 uf (microfarad). This is a simple and effective fix. It dampens RIPPLE from the converter.
Do not wire the polarity backwards. Gob on a glop of silicone over the two screws on the capacitor. No 14 wire is fine.
25 or more volts rating on the capacitor
And it's going to give a brief spark if you do not kill the battery and converter power while installing. Please chop battery power first.