Louder Engine Noise

I have the same symptoms of a very loud droning sound with my 2008 Prius that has 185 k miles. It is not related to RPMs but speed of the car. I can put it in neutral and the sound remains when the car is moving faster than about 30 to 40 miles per hour. It gets louder as I go 60 and 70 mph and is quite uncomfortable now. I've always use the same tires and never experienced this before. I am thinking that it is a front wheel bearing. I would think if there were a hole in the exhaust system it would still be loud at low speed and high RPMs. Is this true?

 

What does it sound like when you punch it from a complete stop? That would get the engine to high revs at a low speed.