Prius (2004-09) :: Low Pitched Roar Or Whine Coming From The Front

About 3-4 months ago, I noticed a low-pitched "roar" or whine coming from the front of my 07, especially at 35mph and above. The pitch of the sound gets higher the faster I go, but its never a high-pitched squeal or anything, just a roaring whine that has gotten somewhat more noticeable as time has gone on. 184,500 miles on the car, Goodyear Assurance tires that will probably need to be replaced fairly soon but are certainly not bald, reasonable tread left. I bought the car with just under 160K, I've put about 25K miles on the tires, but don't know the car mileage when the tires were originally installed.

I had the tires rotated during an oil change about 6 weeks ago, which made little if any difference in the sound. I asked a mechanic I know (and trust) at the non-dealer where the oil change and tire rotation was done, he said to turn slightly left and right while driving to see if the pitch changes, which may indicate a front bearing problem. There was maybe a slight pitch change in the whine when I did that, if any. He said it might be the start of a bearing problem. I also don't notice any change in the pitch or unusual behavior when making turns.

So is this a tire problem or front/rear bearing issue? I'm going to take it to the mechanic for him to test drive, would I be better served going to a dealer? Given the car mileage, it could very well be a bearing problem, it would certainly be due.