A high pitch sound sometimes when braking or beginning to accelerate.
It might help if we knew where the sound was coming from. engine? Brakes?, Suspension?
If it was one or the other you’d likely get a lot of ideas here. But when the sound occurs when first accelerating AND during braking. And only sometimes, that’s a puzzle for sure.
One idea, the suspension system components move both when accelerating and braking. You may have a squeaky bushing for example. If that’s the culprit, your shop tech should be able to narrow down which bushing it is by just pushing on the corners of the car one by one.