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Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.A rattling noise is unusual from a gas valve but noise descriptions are notoriously subjective. If you're sure it's coming from the gas valve and not the system piping then you need to check for 24 vac at the valve. If you have 24 vac at the valve when the noise occurs then you either have a bad gas valve or a bad connection between the valve and the control. Of course if you have a bad connection you're probably going to see something less than 24 volts.
If you're sure all the connections are good then you probably have an issue with the ignition control. What do you have for an ignition control? Is it a Honeywell S86?
I just checked the PurePro specs and it looks like most have a Honeywell S86 so you want to probe across MV & MV/PV. If the voltage is constant then you have a bad lead to the gas valve. If it's not constant probe across 24v & 24v GND. If you have steady voltage there then the control is suspect. If you don't have steady voltage there then you need to follow the circuit back to each component to see where the voltage is dropping out
If 24 v is constant at MV & MV/PV but not at the same terminals on the gas valve then the only explanation is defective leads from the control to the gas valve