If mine, I am sure I could repair the AC for much less than $400. Since you are a member here, I imagine you are hands-on too.

I don't know about the transmission, but since a common problem I expect somebody will step up and come up with a fix. Since it sounds like just within the torque converter, that is a separate part which can be shipped affordably. If RWD, it shouldn't be too hard to remove the transmission to replace, but never seen a Sprinter. You might have to live with the noise for a few years. If from the input shaft in the tranny, it sounds like a worn bushing.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans