Increased engine noise/road noise and decreased MPG?

2012 Prius 3 with 140K miles. Within the past week or so, there is now a noticeable increase in noise that at first I thought might be road noise, but now I think that it is engine or transmission noise. The sound frequency, and to some extent the volume, changes smoothly with the speed of the car. Higher MPH equals higher pitch and somewhat louder, too. It almost seems like the RPM's are too high and the tranny should drop into the next gear, but is this a thing with an ECVT? It's not a rattle of any kind, and I don't notice any difference in response from the gas pedal. Also, we're in the middle of a 1000mi road trip and the MPG's have dropped from typically 51mpg to 45 mpg. What should I suspect?

 

It occurred to me that maybe the car is running in "B" mode (engine braking) instead of Drive, even though the display says that I am in "D". Is this possible?

 

Uphill, downhill, level, cool morning, hot afternoon--- no difference! The noise seems to be tied to the speed of the car, not how hard the engine is working. MPG update: for the next 200 miles of the trip I kept it right at the speed limit (max 70 on highways) and I averaged 55 mpg on that segment. So maybe whatever is causing this noise is not causing a decrease in MPG after all.