I have the same issue. Last year my sodium was 125 in hospital, had salt tablets 3x a day.
Being followed by nephrologist and also restricted to 1 liter of fluids a day.
They said it was from my lung disease.
So far my sodium levels have been 132. I am very strict with fluid intake but you are right, my mucous is thick and difficult to bring up. I always loved water so restriction is difficult. Sometimes I go over 1 liter in a day!