If the cabinet and ground leg of the radio are good, the next guess is that you're getting some unfiltered AC "ripple" in power in. Listen to a flourescent light in a quiet room - that's a 60 cycle hum. If what you hear as static is really AC leakage onto the filtered circuit, it'll sound like it's at the same frequency. Come to think of it, most original Univolts are a BIG 60 cycle hum, and you don't need a quiet room to hear them.
Mine sounds like it's about to levitate, but it's still putting out 13.7 volts, and I try to keep on top of the batteries' water. Replacing it's on the todo list, but it's down a ways.
1975 Argosy 28 "Argosy"
1979 Excella 500 31 "Betsy"
1992 Lincoln Mk 7 LSC
2003 Dodge 2500 Cummins "TowHog"
"Lucy Loosehair" the cat - Airstream mascot
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