can only be one thing
Since the rudder post is stainless, it won't rust which leaves only one other source - the webbing inside the rudder which holds the frame to the post and is welded together. Depending on howq and where you sail, you might consider removing the rudder to open it up for inspection. Once the welds rust and weaken, the possibility exists it could give way under load and if that happens at the wrong time, it could ruin your day.Simply draining trapped water and eliminating the source of water intrusion won't fix the problem