We use the best leather available to make our mandolin straps. So good in fact, that it can sometimes make a squeak as it moves on the strap pin. A number of people have had this problem, so we thought we'd share this fix.
The first thing to try is simply softening the leather around the pin hole by wiggling it back and forth in your fingers, just like you might do to soften a new pair of leather shoes. Another fix is to use a lubricant of some kind, such as leather grease or furniture polish.
Just put a very small amount of polish on your finger and apply that around the inside edge of the strap hole. Problem solved!
Let us know if you need any more help with this.