Oh the prop pitch was definitely the factor in changing the sound, but the sound change was most likely a by-product of the RPM's going ballistic. Prop selection is based mainly on whether you are looking for top speed or pulling power (torque). If you want all out speed you would choose a prop with smaller diameter and larger pitch. If you want torque, choose a prop with larger diameter and less pitch. Whatever prop is chosen needs to be within range for the specific engine so the RPM's at wide open throttle (needle adjusted correctly) are within specs. I would say that a 10x4 is probably too small for your specific engine, 6" pitch would most likely be the limit for a 10" diameter.