I haven't measured mine but there is a fairly good jump from low fan to high fan especially in high altitude mode.Do we have other owners of this projector in the forum who could try the same? Is this level of increase in noise really normal? This is quite an expensive (and huge) unit, i would expect better noise control, it was surprising that previous Sony was so much better in this regard.
One thing you can try, lower the temperature of your theater room. If your projector is ceiling mounted and your room temp is above 70 degrees then your fan could be working more than it needs to be to keep your projector cool.
If you drop your room temp down to say 68 or lower it might seem a little cool in the room but fan noise will drop quite a bit. We live at 8000 feet and I have to run in high altitude mode but our theater is always around 65 degrees and the fan noise isn't too bad. I notice it is louder when our room temp exceeds 70 degrees.