Q: I have a 1995 dodge ram 1500 with a 5.9 L Magnum and it is making this weird noise that started after a heater core exchange

Hi and thanks for contacting YourMechanic. First find all of the vacuum lines that are cracked and replace all of them. Then check the heater control valve to see if it has a vacuum leak on it. With the heater off, the valve should be off and shutting the flow to the heater core off and redirecting the coolant back to the engine. Check the coolant hoses to see if they are all the same temperature with the engine at operating temperature. It is possible that the valve has stopped working and is allowing coolant to pass through the heater core making the coolant temperature lower since the heater core acts as a second radiator. The misfire on the engine could simply be a vacuum line split and causing all of the problems. If all of the vacuum lines are replaced and the engine still misfires, then check the distributor cap, rotor, plug wires, and spark plugs for any wear and burn spots. If there is, then I suggest performing a tune up to fix the misfire. If you need further assistance with your vehicle, then seek out a professional, such as one from YourMechanic, to help you with your coolant temperature and engine misfire issues.