Check the tree of your saddle. There are some really good descriptions of how you can do this on pony club manual ... I just don’t know how to describe it to you! If the tree of your saddle is broken you need to get a new one ! I don’t ride with a broken tree it can hurt the horse and you !
i would get mink oil and oil the saddle inside out. including under all the flaps
it does fit it squeaks even off the horse
it may squeak if it doesn't fit your horse properly, so make sure it fits well :)