Since an electrician is about have him check out the buildings bonding of communication equipment. Old apartment buildings are rarely code compliant with code section 250.94. I live in an older apartment building as well that had horrible ground loops. Here is a couple of videos you can review with the electrician. They will understand the technical requirements.
https://youtu.be/a5zAVo2bvJ8https://youtu.be/NaYJfXmVChQ
https://youtu.be/a5zAVo2bvJ8https://youtu.be/NaYJfXmVChQ