Type of Radiator Leak

Since the vehicle is 14 years old the radiator is just worn out. Radiators today are made of aluminum and have plastic tanks crimped onto the aluminum center part of the radiator with rubber gaskets on the tanks. Each time the engine warms up and cools down the radiator, and other parts of the engine, expand and contract. This puts normal stress on the radiator but over time the rubber gaskets on the tanks deteriorate and can start leaking. Plus the aluminum part of the radiator often corrodes and begins leaking as well. It's normal wear and tear after many years of use. As you drive the cooling system pressures and temperatures go up and down as well depending on the driving conditions and outside temperatures too. Sounds like right now you have a very small leak that shows up when driving at highway speeds when the cooling system is working a little harder. I'd replace the radiator now. The leak will only get worse. If you're in a pinch right now put some black pepper in the cooling system. That's an old Depression era trick that my Father taught me. The pepper will not dissolve or clog up the cooling system. But it will stop a small radiator leak for a while. Hope that helps! Jim