Purity of tone has little to do with mechanical structures being stuck. Open a dead disk drive, tip it 45 degrees, and drop a tiny screw onto the edge of the disk platter.... a beautiful bell-like tone. Even if the heads are stuck to the platters, in which case the ring is dampened to a shorter duration.
samwichse, I think you are exactly correct, dessicant. when scottw removed the box, either he jiggled the platters and that unstuck the heads, or maybe the dessicant box had crept sideways and contacted the platters directly. Eitherway, seems strange to see dessicants inside the chamber. Not impossible, but certainly odd.