You didn't mention your cables..they should be shielded. Its a 2 edge sword, since shielding closes in the sound a bit, but better protects against unwanted radio stations and the like
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I believe that oxidized or dirty contacts can cause that, particularly as here where low level signals are involved that are subsequently subject to high gain. The poor contact results in diode rectification effects, which demodulate the radio signal.
Try using some contact cleaner on all of the connections that are involved, including the ones in the headshell that mate to the cartridge pins, as well as any other connections that are in the path to the phono amp. Or, as a minimum, just slide each of those connections off, and then back on -- that might rub off the oxidation sufficiently.
Re shielding "closing in the sound," I think it's pretty well recognized that very low level background hiss can contribute to a subjective perception of air and ambiance. Good shielding could conceivably reduce hiss that would otherwise be induced by electromagnetic fields from nearby cables or other sources, to a degree that would create a subjective perception of loss of air or ambiance.