Prius Prime sounds like in B gear at all times

There are a couple different sounds that people associate with B mode.

The charging of the traction battery is nearly soundless. (It also happens most times you use the brake, even not in B mode.) You can sometimes notice a faint gear whine from the transmission, just a smidge louder than in normal driving, because of the torque that regeneration is putting on the gears.

The other sound people associate with B mode is the sound of the gasoline engine revving high. That sound is very much like the way any other car sounds if you downshift going down a hill. It is a much more noticeable sound than the faint gear whine when charging the battery. The engine sound comes when B mode is not charging the battery, but has decided to direct the recovered energy into twirling the engine instead, giving the battery a rest.

Which of those sounds more like what you're hearing?

 

Wheel bearing.

 

I agree that it sounds exactly like tire noise. But, you replace all 4 tires.
So, then my guess is a wheel bearing that is going bad.

 

No clue if there's warranty coverage, talk to whomever sold you the warranty.

An extreme failure involves the wheel separating from the vehicle, but usually you get 100,000 miles of howling and groaning noise first. Lots of safety engineered into these. You'd be reading about it in the news otherwise.

They are modular parts meant for relatively easy replacement.

On the other hand they are notoriously difficult to diagnose. Sometimes you can put it up on the lift, roll each of the wheels by hand and it is plain as day. Other times you have to use special tools like Chassis Ears™ and even then it takes an hour of road testing to work out which one of the four has failed.

 

Those descriptions are new to me, but classic descriptions go like: worst tire noise ever, a WWII bomber taking off, just a loud horrible hum or whir... endless creativity in the linguistic end of it.

The big hint here is that it doesn't matter whether your car is running on volts or gas, you're getting the same noise. That pretty much narrows it down to either wheel bearings or tires.

There's a chance you got terrible tires, but there's a better chance you've got a bearing starting to go bad. Time to take it into the shop and let them work it.

The good news is that tires and wheel bearings are exactly as they would be on an ordinary gas car. You don't need Toyota or anyone with hybrid experience to fix this. So if there's no warranty, just take it to the neighborhood mechanic and pocket the savings.

 

Been some time, but that sounded like my HHR with a bad bearing. in addition to the noise, there could be reduced efficiency from more friction in the bearing.

The classic test for a bearing is to jack up the wheel, and see if you can wiggle it in relation to other suspension parts. It didn't with my car though. Couldn't figure out any other causes. Replaced the bearings, and the sound went away. The job is pretty straight forward. If you are comfortable replacing brake rotors, you should be fine DIYing this.

Like tires, its recommended to replace the bearings in pairs.

 

My experience with the ChassisEar involved not nearly so much road testing.

Mind, I had already exhausted every other "just do this! it'll tell you which bearing for sure!" suggestion, then shopped online for a used ChassisEar getup and waited for delivery, and probably spent 20 minutes slithering under the car clipping the pickups to the different locations and making sure the wires wouldn't snag on anything.

But after that prep work, finding the culprit took no longer than driving down the road and having my copilot press the four buttons on the receiver one after the other. May have taken all of ten seconds.

Through the ChassisEar, the channel on the bad bearing sounded like a train derailment.

I didn't replace bearings in pairs. I had two bearings go out on my Gen 1 in all the time I owned it, and those were three years and 50,000 miles apart, and I drove it three more years after that before insurance totaled it after a collision. So my experience doesn't suggest that one bearing wearing out means others are right around the corner. And the amount of work to change two is pretty much just twice the amount to change one, so it's not the kind of job where you say "may as well just do that other one while I'm here". It'll be about the same amount of work if you just do the next one when it needs it, and you might never see that day.

 

I haven't replaced bearings in pairs since the days of annual grease repacking, when you'd have them all apart anyway.

"an hour of road testing" is what's likely to show up on a bill for a non-DIY, and I'd expect that to cover all the mic fitment & cleanup after.

 

wheel bearing in fl kind of surprising, how many miles on her?