UPSchris

I recently bought a house with a Hayward H100ID pool heater and it’s been working fine up until recently. I will plug it in, turn the temperature dial to hot, the blower fan will turn on and then about 5-10 seconds later you will hear a click and it’ll ignite and the water will be nice and hot for only about 5 minutes. After 5 minutes the heat stops but the fan is still running. When everything IS working fine for the first 5 minutes the heater is making like a whistling/screaming sound. This is my first time having a pool and a heater. The heater itself is pretty old. Would anyone know what could possibility be wrong with it? Thank you