It could be a number of things . . . one thought that occurs to me is gravel or something in the volute (chamber around the impeller), and that the gravel is bouncing around on low, but being held in one spot on high.
Cavitation can make a sound like that, but I'd expect it to be noisy on HIGH, rather than low.
If you want to take a video WITH SOUND, and email it to poolforum@gmail.com, I'll give it a listen. Last year, we had a couple of people send videos in, and with both sound and motion, could instantly ID the problem.
. . . membership upgraded.
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