look at the stud on the starter if you havent done so yet. Water tends to trap in the boot and rust the crap out of the connection. I cut a small drain hole at the lowest point and put some grease on there too. The sound you describe makes me think of one of the big cables (ground or+) The ground is easy to reach and check, it only has 2 sides. The + side has two wires (frm the Batt to the starter realy and from the relay to the starter. Check those first.