Bought a 2005 Prius with a perfect history and that recently had its 60k check up at the Toyota dealer. Don't know exactly when the sound changed since I'm not the primary driver, but now it sounds like a jet airplane when it reaches highway speeds and it's hard to hear what we're saying to each other, much less what people are saying over the Bluetooth (although the built in Bluetooth noise cancelation feature is fantastic and the person on the other end of the phone doesn't hear the noisy engine). Any idea what could be causing the car to sound like we're taking off at the airport (at any speeds above 40 mph, it seems)? Our passengers comment on the near constant background noise level.
"home' seems vague, but if you are in the mountains, the Prius can use the Engine as a brake when the HV Battery is too full to accept any more power. Were you in a long downhill?
Check the side mirrors. There are screws on the bottom that should be covered with small caps. If the caps are missing you will hear a whistling noise at highway speeds. Just put some tape over the screw holes and the noise will disappear.
Is it possible that your car has a bad wheel bearing? It sounds like this problem is easily demonstrated, so I suggest you take the car in for service soon.
I agree with Patrick. Is the noise more on one side of the car over the other? If so I am willing to bet it is a bad bearing.
Poor wheel alignment can sometimes sound like a bad bearing, and if the tire(s) has cupped or otherwise become worn, it can get noisy in a hurry, especially if it is a front tire.