Thanks for the response! I’m familiar with electro-magnetic fields, and in fact have an EMF meter that I use to monitor EMF fields in my apartment. Such fields are certainly present, but they are not unusually strong (at least, not unusually strong for an NYC apartment!)
I hear the hum in telecoil mode everywhere in my apartment, everywhere in the public areas of my apartment building, and everywhere in the streets within a 4 block radius or so - I haven’t checked further than that. It does sound as though I’m picking up EMF fields - the signal is strongly directional, and if I tilt my head so that my ears (and the hearing aids) are parallel with the floor or street, the signal generally nulls out and the hum disappears.
I’m wondering whether there is some sort of filtering setting within the telecoil mode that is improperly set - it’s hard to believe the telecoil is working properly since the hum makes it unusable in practically every public and private space I’ve tried them in.