For the record, this EXACT same thing happened to me a few times on a 300 mile drive (each direction). Sometimes it was ridiculously loud, like almost hurt your ears loud. At first I thought it was another car or something. Then I thought it might be the way my new tires were interacting with road grooves or something. But then I realized it only happened on inclines when the engine was straining. It's almost like the entire car becomes a bell and is vibrating a sound. High pitched (but not overly), comes on steadily until it hits a peak, and then lets up when the engine relaxes. No other symptoms. No loss in power, no real vibrations or anything - just a freaky sound. Sometimes it would be louder than others. And again it happened maybe 3 times each way - it's not a common thing.
Anyway maybe this will help someone else, or if other people see this we can try to isolate what's going on. I have a 2019 Energi.
