The noise is from the belt. Think of it like a guitar string. Small vibrations from the motor get played by the belt like music. Some of the noise is also from the vibration of the motor itself but it is basically getting amplified by the belt. Change the tension and you change the pitch. That's also the reason you can notice differences in the sound based on how hard you twist the throttle...under higher torque the motor vibrates a little more and the belt will be tightened giving a higher pitch.
I have actually ridden enough that I can tell a belt that is over tightened just by the sound. There are apps that check belt tension based on this principle. I can also tell the difference between the older 14mm wide belt and the newer 17mm wide belts that come on most of the 2017 lineup. As you might expect the wider belt has a little bit of a deeper pitch.
The question mark is why do you notice a difference just from it being a rainy day. I have definitely noticed this same phenomenon many times....as recently as just 30 minutes ago on my rainy ride home in fact. I have two theories but haven't been able to confirm if either are correct of even possibly a combination of the two.
1. The changes in temperature and humidity cause the belt to swell which results in a tighter belt and the change in noise.
2. The addition of the water on the belt changes the pitch by itself....playing a wet string vs. a dry string.
I have thought of ways to test but haven't done so yet.
It gives a new meaning to tuning your bike