Hum in disconnected line

[Jack Aman]

Here is a stumper, but maybe not for the brains here.  I use an Xlink bluetooth device in my home.  All calls come and go via my cell, but the Xlink feeds the house wiring and drives a couple of 202's and a couple of 302's as well as a modern three-station wireless.  It works remarkably well and the house phones act just like POTS.  All the old phones even dial out...the XLink "understands" pulse.  When I work on one of my old phones, rather than crawl behind furniture to unplug, I unplug the line at the Xlink.  I know the house wiring is isolated at the box because I unhooked it, and even if it were not, the previous owner of this house disconnected from POTS by taking a pair of dykes to the drop!  No question the house phone wiring is isolated.  Npnetheless, when I have the house lines off the xlink, there is a faint hum in the earpieces of all phones.  No question there is life in these lines.  Where can it be coming from? Could my Panasonic wireless base be feeding something into the house wiring even when those phones are "on hook"?  It doesn't effect function in any way, but I was surprised to hear it today.

As always thanks in advance for interesting insights.

[G-Man]

 A faint hum is not to be unexpected. Most likely picking up 60~ AC (inductively) from your household electrical system.

[AE_Collector]

If the drop is chopped off of the prot then the station wires are likely still connected together at the prot which also likely has a ground connected to it. So the possibilty of a partial ground on one side or the other exists as well.

I would disconnect one thing at a time from the phone jacks listening for a change in the hum and if that doesn't Identify a culprit inside then try lifting the station wires off the prot and just connect each side of the station runs together in some fashion.

Terry

[andre_janew]

Do you have any cordless phones hooked up?  If so, that may be where it is coming from.

[twocvbloke]

It reminds me of when I was messing about with one of my GPO 746 phones, I had previously fitted it with an electronic transmitter and then went to use the phone, noticing there was no dial tone but there was a mains hum, and upon discovering the thing wasn't even plugged in, it was highly confusing, it wasn't until I was reading about the No. 21a transmitters on Bob Freshwater's britishtelephones site that I found the transmitters I had were first versions that had mains hum issues and later ones were modified with shielding round the transmitter's PCB, so I modified mine with foam padding and tinfoil, and the hum was gone...