Take a look at the "J" bend in the spoke, found at the hub flange. It may be about to break.
Also look for small cracks in the rim near the spoke holes. This is where rims usually fail. If there are any cracks the rim needs replacing.
It's quite possible that you do not have enough tension in the spokes. Because of the rear wheel's offset hub flanges the non-drive side spokes have the lowest tension and if too low will break at the "J" bend due to excess flexure.