Try baby powder.
When leather creaks/squeaks, I use baby powder to quiet it down. Seems to work okay with kydex too (as long as you don't mind a bit of baby powder on your gun).
Squeeze some inside the holster and then shake it vigorously, you just need a light coating to quiet the pop. Imagine looking inside a black holster and seeing a thin white veneer over it. It just provides a dry lubricant for helping slide things a long, without being particularly abrasive. (Makes sens, no? What's it for if not for helping prevent rashes by helping displace moisture and reduce chaffing...)
If it isn't to your liking, just wash the holster out in the sink.