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|I agree that a well-maintained J-H with such low mileage is unlikely to have internal bearing problems. So my next guess would be external parts which tend to degrade with time rather than mileage, such as the engine dampener, rubber engine mounts and rubber and plastic parts on the intake.
What is interesting is that the noise is greatest when revved with no load. This is the situation where the engine will twist the most. If you have the original Stromberg setup, one possibility is the hose between the airbox and the air filter is loose or has a slit in it which opens up when the engine tilts over.