Sprinkle baby talcum powder or silicone spray underneath the sole if the shoes are dry. If you think the noise is coming from friction, try rubbing sandpaper on the sole. You can use leather oil or petroleum jelly also.  Additionally, if you have the right fit, you'll be able to walk quietly. Consider switching to synthetic material socks if you're currently wearing cotton socks. Socks made of cotton have been known to generate farting noises.