When I have my laptop at my desk, I plug in my computer speakers to it. When I move it to my bed, I have another headphone cord that runs there, which is also plugged into the speakers. The problem is that when that first cord is sitting unplugged, but with the other end plugged into the speakers, it of course generates a considerable hum.
I notice the hum goes away when you plug it into any jack, which makes me wonder: is there some kind of jack-shaped "cover" one can put on it to stop the hum when you're not using it?
The alternative would be to unplug that wire from the speakers each time, which is inconvenient.