Apologies to the group if this has been discussed prior:

When I plug my subwoofer into my receiver and turn on the receiver, I
hear a hum. The sound is similar to putting a finger across the
subwoofer cable lead.

I am trying to understand if there is an obvious cause for this hum,
other than a problem with the receiver's subwoofer output jack. I do
not believe this is a ground loop issue.

My equipment is an Onkyo TX-SR500 receiver and Sony SA-WM20 subwoofer.
(I know, not the greatest - but it's what I have.) I tested the
subwoofer on a friend's Onkyo, and it worked properly. I swapped
subwoofer cables and the hum still occurs.

I have disconnected all input/output leads from the receiver other
than the subwoofer cable. When the receiver is not plugged in, there
is a low hum. When the subwoofer is plugged in, the volume of the hum
increases, even with the receiver power off. When the receiver is
powered on, the hum's volume increases more. The receiver and the
subwoofer are configured to plug into different grounded outlets.
Both outlets are properly wired.

The system was working properly before I pulled it apart to install a
wall unit. Other than upgrading some RCA cables and changing the
outlets into which some components plug, no changes were made to the
system. When I disconnected the system, the subwoofer cable was very
difficult to remove from the receiver, In fact, the head came apart
in my hand, and I needed to use needlenose pliers to pull the casing
off the receiver output jack. I am wondering if I somehow damaged
whatever is hooked to the backside of this jack, and whether that
might cause the hum.

Any advice before I cart the receiver off to a shop is gratefully
accepted.

Brian