I have a rust stain down on the boot stripe that is coming from the common drain on the Starboard side of the boat. This is the thru hull drain that is just forward of the A/C thru hull drain. I have had stains from the A/C drain before but never this one. I have never actually seen water coming from this thru hull drain. I cleaned the rust stains off and reapplied polish about 2 weeks ago and now it is back. What is connected to this common drain?