Some saddles just do that for some unknown reason... I know it can be very annoying. See if you can work out exactly where it is coming from i.e. stirrups, seat etc. Then you might be able to do something about it.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare