I agree with Eagleearl, and the squealing you hear could be either a wet belt slipping on a pulley, or the bearing in the water pump complaining about being full of coolant instead of grease. I bet the water pump is failing.
Do yourself a favor and check the oil to see if you have done any damage to the engine - if the head gasket failed from overheating, the oil will have coolant in it and look like milkshake. Look in the oil fill hole, as well as the dipstick, or change the oil if you're due.
Also, when filing a cooling system, never use pure water unless its an emergency, always use a 50/50 mix of antifreeze and filtered/demineralized water - you don't want to use well water/hard water or municipal supply, since thery're usually full of minerals that will build up scaler inside the radiator and heater core. 50/50 mix coolant is better at absorbing heat, since the antifreeze raises the boiling point of the water when its mixed into solution. It also prevents corrosion, where straight water, when heated to near its boiling point, is a decent corrosive agent. That's why steam cleaning stuff like carpets works so well.
Finally, if your cooling system has even a tiny weep of a leak anywhere, its losing pressure, and pressure in the cooling system prevents the coolant from boiling. Raising the pressure of a closed system containing a fluid will raise the boiling point of the fluid. Fix all leaks, no matter how small, and the radiator cap can be a leak source - its supposed to hold up to the pressure its rated to.