I actually have an unrepaired fault in this area. Don't have a CEL, but do have code B1239 (Air Flow Blend Door Driver Circuit Failure). I looked into it at one point when I had some time, foolishly thought it would be a good idea to hunt down and fix all my codes on all my cars, made little progress and sounded the retreat. I'd have tried further if it mattered to me. So anyway, ...
Have you checked for codes? As with me, I had no CEL, but my Foxwell NT520 came back with the B1239. If you can find a code, and then put it into the Google intertube machine, you might get more info to sort through (which might not help any

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My problem was definitely different vs. yours since mine was silent and throws a code. But I believe it is the same part. My code indicates circuit failure rather than a mechanical failure.
Everything
@phattyduck is saying here make sense to me, and I will reference it next time I am "in there".
I found a couple of threads on here, when searching on that B1239 code:
That second thread has a possibly useful video in there, aiming to show what his noise is like. Unfortunately I think he was driving through a hurricane for the first half of it, but reached the eye part way through.
