1970 Datsun 240Z — humming rises with road speed, not engine RPM

Write down what you already tried. A second data point from another 1970 Datsun 240Z owner helps more than a generic diagnosis. I'd do that before spending real money. This is from a 1970 Datsun 240Z in Long Island with 247,000 miles. If it doesn't change the out-the-door or the repair estimate, it's noise. I'd sleep better after that step, even if it finds nothing. This is the version I'd tell a friend.