I've been driving this car since the Summer of 2000 with 217k on the clock. Until the past year I never put more than 34 lb in the tires. And the tires I bought before then were not ever rated for the proper load range, the local shop fitted the proper size and didn't acknowledge the Prius tire specs. I've averaged 47-50 all these years. I am aware of the changes in mileage according to the seasons. The mileage drop pretty much happened soon after the hybrid battery change a year ago. Unfortunately, I got new tires a week after the battery so I can't pin point exactly when it happened. The didn't start until a month ago.
If the sound was coming from the tires it would be a much higher pitch OR I would feel it.
Maybe I'll get enough of a bonus or Christmas money to have it checked out after the holidays.
Goin' for 500K!