update if anyone cares. turns out i had a significant leak in my rear left door element. The physical source of the sound, the long whine that lets out when I had the car in unlocked position after turning off engine, was playing tricks on me. So hard to tell where it was coming from - but I finally tested all the lines again and realized there was a huge leak in that door - must have missed this while testing last year, or it developed/got worse. replaced that actuator and now lock system is working very well. probably won't hold vacuum over the course of a couple nights, but good enough for me.

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Andrew
Montclair, NJ
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1982 300D Turbo
120k
Petrol Blue Green