A couple possible solutions:
Might be the leather squeaking against the rear support. Smoosh a little bit of Proofhide/SnoSeal/Obenauf's LP up in there and see if that stops it.
Might be the rails creaking in the post clamp. The rails flex as the leather 'gives'; I'm not saying open the clamp and grease 'em up, but a single drop of 3-in-1 at the rear edge of the clamp where the rails come out may help.
This was the issue I had with mine, and I had a 2 part solution: 3-in-1 on the edge of those rails, and I punched/laced the skirts to reduce the overall saddle flex.
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"I feel like my world was classier before I found cyclocross."
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