Low-pitch hum in tight low speed turns

I recently test drove a used 2017 Subaru Impreza and noticed a strange behavior I haven’t encountered before, and that the mechanics at the dealer could not explain.

With the car stationary, if I turned the steering wheel fully to either side, once the wheel was at its limit, a very steady low-pitch hum would develop, almost like an audio base test tone. The radio was off during this testing, so I don’t think it was a feedback issue through the speakers.

It was a very steady base note, not a rattle/vibration/clunking sound like I’ve seen many people describe with other steering/turning problems. Because it was quite low-pitched, it was hard to tell where it came from. It felt like the whole car was emitting this sound.

If I then drove in a tight low-speed turn with the steering wheel all the way to one side, the base note I was hearing would get louder, but would not otherwise change in character, i.e. it didn’t then change to a rattle or knocking. If I backed the steering wheel off of its limit, making the turn a little less tight, the hum would go away.

There were no other obvious mechanical issues with this steering procedure, i.e. the card wasn’t lurching or shaking during this low speed turn test.

I do this tight low-speed turn test on every car I drive. I’ve recently test driven two other Imprezas and four Crosstreks (almost the same car but raised 3"), all similar years and mileage to this car, and this did not happen on any others so it’s not just a standard character of the vehicle or its steering.

Thanks in advance for any theories on what this noise was.