The expansion tank for the coolant is leaking.
The expansion tank on these vehicles are known to have leaks after years of use. The new replacement tanks hold up better to the coolant heat and pressures they encounter. You will most likely need to get the entire tank replaced with a new cap. I also recommend getting the complete cooling system checked for any other issues before replacement of the tank. If you would like to have this done, have a certified mechanic, such as one from YourMechanic, come to your home or office to locate the leak and make the necessary repairs.