Just put back into my 55 the heater with a new core and valve. When the heater valve is off and I rev the engine, I hear a chatter or shuttering noise in the heater housing I believe from the valve. Would reversing the heater hoses help? Is the coolant putting pressure on the backside of the valve because coolant is pushing on it in the opposite direction? If I open the heater valve, the noise goes away entirely.