I feel like it has to be some sort of busted valve because of the the noise characteristics (or actuator, but in this case it's happening before any actuators). I don't think a cracked hard line would make this kind of whine, it would be fairly silent because there is nothing much to vibrate. This whine has a distinct rubber/diaphragm character to the sound, if that makes any sense - but I could be wrong.

I'm fairly certain there is no check valve in my door, I've been in there once already to test the master element.

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