I took the car to my favorite mechanic, and it turned out to be the right rear wheel bearings. He said they were very corroded and it took quite a bit of effort to get it changed out.
When my wife (who drives it the most) got in it she said it's never been that quiet, so I suspect the problem was already starting when we bought the car used a year and a half ago.
As far as the new tires, obviously it was happening way before I had them put on. I noticed however that the wheels on this car are difficult to get off because of that center hub thingy, and I had the tires put on at... walmart. They were none too careful in removing the wheels, and I think that made it even worse.
Anyway it's working like a charm now, and I can only hope that the remaining bearings won't follow suit.