Wheel bearings or tires? (humming)

I have 14 Prius 2 with 147K. Last week I started to hear a hum around 35mph and a little louder at higher speeds. This past weekend I replaced the rear driver side bearing since it seemed loudest from that corner. Took it for a ride and the noise is still there. Replaced the front driver side bearing and the noise is still there. I'm starting to think that maybe a belt let loose on one of the tires. Only because I originally thought it was the rear bearing and was wrong. I drive about 600 miles a week and now am not sure what is wrong.
Thoughts?

 

Where would you suggest I get a free inspection at?
I thought someone else might have gone through what I am and had some helpful insight.

 

Thanks for the info. I rotate my tires every 5K when I change the oil. And I have looked at the passenger side but nothing out of the ordinary.
I'll put the fronts on the back and see if the noise is greater in the back or the same. I guess at 147K, replacing the other 2 bearings isn't such a problem. The car runs excellent otherwise.

 

The rear driver side seemed pretty tight, since I already bought the new one, I thought it best just to replace it since I have 147K on it.
The front driver side had some play in it and I was convinced this was the troublemaker. But it's still making a (hum) when driving. I guess it would be best just to replace the passenger side as well. Getting expensive though but I shouldn't complain, this is the first issue with this Prius.