Grinding noise when coasting, hybrid battery fully charged

My car is a 2020 Prius AWD. I live in a mountainous area and hike a lot. When driving down a narrow mountain road (paved) where I am coasting and braking around corners, I start to hear a grinding noise when the display indicates that the hybrid battery is fully charged. The display also shows the charging arrow traveling from the front wheels to the inverter and to the hybrid battery - even when the battery is full. I have made a recording of the grinding noise and taken it to the dealer but they can't seem to figure it out. As soon as I step on the accelerator, the noise stops and of course the display shows energy transferred from the battery to the front wheels. Due to the nature of the road however, I can't accelerate very often. The dealer is unable to recreate the noise since it doesn't happen in routine city driving. Prior to having this car, I had a 2010 Prius that I bought new and drove for 150,000 trouble free miles. It was still running like new when I traded it for my current Prius. My 2010 Prius never made a grinding noise. I hope someone can help me figure this out. Thank you!

 

55,000 miles. I'm not braking too much - mainly coasting. The noise only happens once the hybrid battery is full. As soon as I step on the accelerator, the noise stops. The grinding noise seems to be more prevalent when I'm coasting. I will try shifting into B and see what happens..

 

Okay, so today I took my car to the top of a winding, steep paved mountain road and shifted into B to come down. Everything was fine at first although I did notice that it seemed to take longer than usual for the hybrid battery to indicate full in the display. I only had to step on the brakes occasionally when going around a steep curve. However, as soon as the hybrid battery was full, the grinding noise started and it was worse than ever. It really sounded like something was about to break so I put it back into D and that helped. This is only happening when the hybrid battery is full so I really think it has something to do with that battery. In normal city driving, the car runs perfectly. Since I hike 4 or 5 times a week though, I want to try to resolve this problem. One question I have - When the hybrid battery is full, shouldn't there be some kind of a feedback loop that prevents excess energy, from coasting or braking for example, from being forced into the full battery? If I step on the accelerator, the noise stops instantly.

 

Thank you. I'll see if I can get a tech to go with me. One of the trailheads is only about a six mile drive from the dealership. Is the ECU the electronic control unit?