Sorry, I inadvertently commented on your post and then had to delete it, John.

I think the Parrot of Doom instructions do not address the potential electrical failure of the actuator. In my car the electrical actuator acted like it was "tired" and would default to the open position on the driver's side. The instructions get the actuator off, so once it is off you can test it for continuity (Ohms) and possibly inductance. Not sure what the results should be, but you can start by comparing it to the side that seems to work all the time. I was not sure how to determine if the control system was at fault or the actuator was at fault. In the end the dealer replaced the duovalve and since then the system works fine.

Also, that hissing noise is pretty normal as far as I know. In the evaporator the liquid freon is given the opportunity to boil off as it comes out of the valve and sprays onto the heat exchange surfaces of the air cooling unit. This causes the hissing noise you hear as the heat exchange surface is in direct contact with the air venting into the car, and the surface is a very thin membrane to be more efficient. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)