I doubt very much that it's a valve spring. A weak spring will show up as a higher speed misfire, assuming it doesn't allow the valve to drop into the cylinder and destroy the engine.
More than likely it's a loose piston, from a collapsed skirt. For a B23 to exhibit that symptom it was probably overheated or run low on oil. Might be a piston pin, if the sound is more of a "tinkle" in nature, and a bit faster than idle speed. Not worth trying to fix, just monitor it and keep good oil and coolant in it.