FWIW if all else fails the noise may well be caused by the airflow over the small drainage holes in the front fork legs. These can generate an edgetone in exactly the same way that a flute produces sound. My bike has a steel fork which whistles quite well, and I assume that your elderly bike has a steel fork also. To determine whether the fork is the cause stick bits of tape over the holes to blank them off. I did, and mine went dead quiet. Could be your answer too. Whether you leave them blanked off is your decision.

Cheers