I forgot to suggest a couple of things for your observations:
1.)Check your readings again with your isolation TX unplugged. It helps you see how much your setup is acting like an antenna. If the readings are non-zero(They will be.), try moving the leads around on the table or placing your hands along the wire insulation in various places.
2.) Switch your meter to read frequency if it has that feature. See if you're reading line frequency or a harmonic of same.
Joe
