Ivan reminded me that I had a similar problem. Stated less well than he did, the wires from the internal amplifier to the speaker terminals had basically shifted inside the subwoofer, after about 15 years of use. So the wires actually rested close to or against the speaker cone. And only when played at loud volume they made a sound that was more like a rattle than what it really was. Opened it up, shifted the wire away from the cone and perfect.
George
George