'B' Mode screeching (metal grating) Noise on long 12 mile descent

A 2011 Prius with 189K miles on it and using 'B' mode on a long descent caused it to make a loud, medium pitch screeching / metal against metal grating type of noise.

Was in Glacier National Park , "Going to the Sun Road", traveling west (from Logan Pass Visitor Center) downhill, switched to 'B' Mode on the descent. Speed modulated between 22 and 30 mph. On reaching "The Loop" close to Crystal Point (about 12 miles from where 'B' mode was engaged), the engine or transmission (was not in a position to stop and isolate the noise), starting making the above noise. The only way to stop the noise was to take the car of out 'B' mode and put into 'D' mode. I was able to replicate the above situation pretty accurately on two different days.

Car did run / drive fine without incident after switching to 'D' mode, though I did not use 'B' mode for at least half an hour after the above incidents.

Since then I have traveled another 3000+ miles (down to Moab, Canyonlands, Mesa Verde) and have not had any driveability or noise issues when using 'B' mode on the Interstate or State Hwys. However, none of the Interstate or State Hwy downhill sections are more than a few miles.

Questions:

1. Is there a mileage limit for which 'B' mode was designed or is there something else going on ?
2. What may be the source of that noise ?

Thank you.

 

Thanks for your comments. Instead of replying to each one maybe I list my responses here:

1. I am familiar with the noise the engine makes when in 'B' . It was not that noise. Think of gears grinding / clashing noise. Difficult to take a recording of the sound under the circumstances (if you are familiar with the "Going to the Sun Road": traffic, curves, incline). 'B' mode was perfect to descend to this road and keep within the speed limit while still being able to take in the views. Engine made it's usual 'B' mode (you are killing me whine) until around the 11 or 12 mile marker when the grinding / grating / clashing noise started. Fortunately I was able to replicate that on two different days .
2. I was not riding the brakes and foot was not on the brake pedal.
   
3. At 180K was the last full inspection (at Toyota dealership) and brake pads were not an issue . (If I recall I had quite a ways to go on the pads remaining)
   
4. Transmission fluid was changed at 150K and no issues were reported then.
   
5. All work done at Toyota Dealership only.

I am wondering if something in the engine / drivetrain heats up in 'B' mode after 11 or 12 miles of use or if 'B' mode was not designed for usage of such continuous distance?

 

I don't recall checking that.

 

Transmission oil changed at 150K. Brakes inspected at 180K.

 

Thanks all. Car returned from dealership yesterday and they could not find anything of issue. Called them and gave them some of the suggestions from Priuschat and they could not find anything. I have a trip to Anchorage coming up next in this old car and fingers crossed. (At least its not terribly cold this time of year)

Unrelated to priuschat, but if outdoors is your thing, I highly recommend a visit to Glacier National Park (aka, the Crown of the Continent) and any car with engine braking is a perfect vehicle to help you safely get around while enjoying the gorgeous area. Not sure what my experience would have been if I was in a car where I had to constantly ride my brakes.