I don’t know if this has anything to do with the Las Vegas trip I just took because it was hot like 118 degrees.There is also a fuse back there for the traction battery that has been known to fail. I think there is a recall for any car showing symptoms. Such a noise is cause for concern and I'd have the dealer evaluate it. Not a safety concern though, more of a dead car issue with more hassle to deal with while it (and you) are immobile.
Could be a fan motor issue too, at least if the start algorithms test it every time the car is turned on. If there is a battery heat sensor that is reporting a high temperature requiring the fan, that also could be concerning.
Yes I haves the same problem and now my car KIA Niro PHEV 2019 left me on the street 3 times already the battery die and I have to tow the car. I can’t even travel because I don’t fell confident.So the other night when I was inside my car, I started hearing this low buzzing sound that seemed to come from the right rear seat. I have never heard of this noise before. Has anyone had something similar to this?