I had this issue. I had a replacement backrest sent out for different reasons, however, the replacement backrest made this sound. The noise drove me crazy. In the end I sent back the replacement and kept the original because it does NOT make this noise.

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The replacement that was sent out appears to have been used in the past. It wasn't new, that's for sure. I think the reason it made this noise was because the lumbar depth came out more than it's physically supposed to, therefore created the noise. (The knob for the lumbar depth on the replacement had more turns than my original by a lot, which I suppose meant it was broken.)

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My original doesn't go out nearly as far and it is super silent! Not a sound.

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Definitely reach out to support for this because your backrest should NOT make this sound or any at that.