I just wanted to post an update to see if anyone can shed a little light on the problem. The 'clicking' occurs as soon as the car is up to cabin temperature. It is always at regular 4-5 second intervals. If I have the thermostat at a moderate cabin temperature it will continue to click non-stop once at temperature. If I crank the heat up the clicking will stop for a while until the car gets back up to the higher requested temp. I'm assuming this is the valve's normal response for the increase in demand for more heat. The the more heat the heater core demands, the more the valve lets through. This is pointing me to believe that the valve is just louder than it should be (why I have no idea). Keep me posted!

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1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL