Anyone has any insights or ideas as to what the issue may be?
Yes. Your turntable has a preamp built in. When powered off, any preamp can exhibit a buzz. Get rid of the wire, which is causing noise to be picked up, and turn the speakers off when not in use.
So
probably the cartridge is picking up hum from some other equipment in
the vicinity. Try moving the table, rotate it 90 degrees or move it
further from the other stuff.
If this is any chance a Grado cartridge, they are notorious for hum.
The reason there is a buzz is that the tonearm is not grounded because the ground wire got torn off or removed. So a spot connected to the arm itself should be found for the wire and the wire in turn grounded to the preamp. This will maintain the grounding/shielding system of which the tonearm tube is a part.
