You finish a piece, it looks exactly how you wanted, and then you pick it up and it creaks like a old door hinge. Nobody warns you about this part. It's one of the most common "wait, is this normal?" moments in leatherwork, and the honest answer is: usually yes, and it's fixable.
Here's what's actually causing it, because the cause changes what actually fixes it.
It's probably one of three things
1. The leather hasn't broken in yet. Vegetable-tanned leather especially is stiff when it's new. The fibers haven't relaxed, so when two surfaces flex against each other, they don't glide, they grip and release, which is what makes that creaking sound. This is completely normal and usually fades with regular use as the fibers loosen up.
2. Two leather surfaces are rubbing directly against each other. A folded wallet, a strap against a bag body, a flap closing over a pocket. Dry leather-on-leather contact is naturally noisy. It's not a flaw in the piece, it's just physics: two rough, dry surfaces sliding against each other make sound.
3. The leather is dry. Leather that hasn't been conditioned loses some of its natural give, and dry fibers rub louder than supple ones. If a piece has been sitting unused for a while, or lives somewhere hot and dry, this is often the main culprit.
One thing worth ruling out before you blame the leather at all: check the hardware. A rivet, buckle, or snap that's slightly loose can squeak on its own, and it's easy to mistake that for the leather itself.
What actually fixes each one
For new, stiff leather: just use it. Flex the fold points by hand a few dozen times if you don't want to wait for regular use to do it. This is the one case where the fix is patience, not product.
For leather-on-leather friction: a very light dusting of cornstarch or unscented talcum powder on the contact points is an old trick that works because it reduces direct friction between the two surfaces. Wipe off the excess so it doesn't build up visibly.
For dryness: a proper leather conditioner, applied thinly and evenly, restores enough suppleness to quiet things down. Don't overdo it. Too much conditioner darkens the leather and can leave it feeling greasy, so a thin coat, left to absorb, beats a heavy one.
For hardware: a tiny drop of oil at the pivot point of a buckle or clasp, or simply tightening a rivet, and the mystery is usually solved without touching the leather at all.
Why this catches people off guard
Nobody photographs the noise a piece makes, so it never comes up until you're holding your own. It's not a sign you did something wrong, it's just what leather does before it's been broken in or conditioned. A piece that's precisely built, with pieces that actually fit together the way they're supposed to instead of leather rubbing where it shouldn't, tends to settle into quiet a lot faster than one where the fit was a guess to begin with.
If getting a clean, accurate fit from the start matters to you, that's exactly what a properly measured, labeled pattern is for. Check out the Master Leather Pattern Collection if you're starting your next build.