2019 Prius loud high pitched noise when starting the car

Does it occur only when your iPhone is in the car?
That might explain why does not happen when you are not in the car.
Try turning off the iPhone so it doesn't pair to the car and see if you can create the sound.

 

We have exactly the same problem in our 2019 Prius. Happens about every 4th to 8th start so it's easily reproduced except that you can't just do it 4 to 8 times in a row. You need to wait at least a few minutes between each start. Was just about to make an appointment at the dealer since we have a zero deductible platinum Toyota warranty. The problem goes away entirely when you pull the SOS fuse but then the bluetooth microphone stops working so that's not a long term solution.

Also, when it happens, if you stick it into drive or reverse really fast, it stops it immediately, otherwise it runs about 2 to 3 seconds in length.

I searched about a year ago and no reports of it so I thought it must be rare but now I see you're having the same problem.

 

BTW, I noticed you have a power cable run in the headliner or a dashcam. We do as well. I'm certain the dealer with blame the problem on this first. I'll need to remove the cable prior to bringing it in.

 

Your SOS subscription is not active. If you renew it, the problem will go away. After some additional research, I've found other instances with other Toyotas. Same problem but happened when their SOS subscription expired and then stopped when they renewed. Our also started in September which was right around the time that Toyota sent us multiple renewal requests that our 3 years had just expired.

2019 Highlander Platinum - painful ear screech sound from speakers when starting car | Page 2 | Toyota Nation Forum

http://www.reddit.com/r/Toyota/comments/vfli4h/high_pitched_noise_when_turning_on_engine/

 

I have a 2022 and this happens randomly. Loud piercing sound that lasts 2 to 3 seconds. I swear my ears are damaged. Dealer won’t do sh*t since they can’t reproduce it on the spot.