It's tough when you're touring and doing your own work, because you don't have a tensiometer. I agree with rhm, that it's almost certainly too low spoke tension. Your drive side spokes are under more tension, so are stretched more, so don't loosen. Straight gauge spokes are more difficult to get just right, because they don't stretch as much, so there's a greater tension change in a smaller length change.

All your spokes on the non drive side are probably fatigued and should all be replaced. Easiest to do them all at once. I'd replace all of the spokes on both sides with the same, butted spokes - they're cheap, so replace the new ones, too, and tension up the whole wheel. Your NDS spokes want to be at least as tight as your front spokes, or better, find a shop with a tensiometer.