I normally run either a full-grain leather Kore Essentials belt for a lightweight IWB or one of their "tactical" nylon ones if I am going to carry something heavy and/or OWB. I find the leather to be totally quiet with a Kydex shell and polymer clips. The nylon ones tend to be more noisy, and for those I will occasionally apply a minimal amount 3M silicone lubricant with Teflon to the top and bottom edges, which usually kills the noise, at least for a while. I mean a minimal amount, applied from a dripper bottle onto a q-tip, not hosing it down with a spray can. For leather I'd probably be concerned about using silicone... maybe a small bit of Obenauf's oil or something instead. I condition a lot of my leather stuff with Obenauf's, including my gun belt, with good success. You don't want to use a lot of it on a leather holster to avoid softening it and ruining your retention, but a little bit on your loops and other contact points with the belt would probably be alright.
