Intermittent drone from front--what's the cause?

I have a 2007 Prius with 180,000 miles, bought new & dealer-maintained ever since, which has recently developed an intermittent drone or growl from the front of the car. When it occurs, the noise varies with the speed of the car, from a low growl at low speed to a howl at highway speeds. It starts & stops without any apparent cause. So far, it happens infrequently, perhaps a few times a week. And so far, it usually stops after just a few seconds or minutes. All the drivetrain parts are original and have never been replaced, including the U-joints, driveshafts, and CV "transmission." We did have all struts & shocks replaced when the dealer recommended it. The car has never been in an accident.

No warning messages have ever appeared in the car's dashboard display, even when the drone sound is there.

The only time this drone was constant was when I had a flat from a very bad pothole and had to put the mini-spare on the front. It seemed to be caused by the relative speed difference between the the normal Michelin tire on one side, and the mini-spare on the other due to slightly different tire diameters. The drone ceased once I got new Michelins all-around, but more recently it's started to occur briefly from time to time.

Has anyone had similar symptoms? Does anyone know what's causing them? Does something need to be replaced?

This is the best car we've ever had, by far, and we intend to shoot for 300,000 miles. But we have some long-distance trips this summer of a few thousand miles each, so I need the car to be reliable. This is our only car.
Thanks for any help or advice you can offer--I've always found Priuschat and its members to be a fantastic resource.
Bill in Michigan

 

Tombukt2's suggestion of bad wheel bearings jogged my memory that I did have both of the front ones replaced long ago, when they were making a rubbing noise. So, that might be the problem, if I'm lucky. Cost of replacement was not exorbitant from what I recall.

Will take it to my dealer and see if it will make the new noise, and will let folks know what the mechanic says.

 

Checked my records, and see that I had the front wheel bearings replaced by my Toyota dealer at 108,837 miles--current mileage is 181,000. So, it seems a bit early for these to have worn out. Have dealer appointment Tuesday next week to see if they can diagnose this strangely intermittent noise. Assuming it makes the noise for the dealer...

Would welcome thoughts from anyone else about the possible cause of this noise and fix for it.

Sound ranges from a growl at low speeds to a howl at highway speeds--but it only comes ocassionally (a few times a week) and it starts & stops instantly. It usually stops after just a few seconds or a few minutes at most.

We bought our 2007 Prius new and have had it dealer-maintained ever since.