I do hear a slight buzz coming from the video but if you don’t hear the buzz from the instrument then odds are it’s how the microphone on your iPhone is picking things up. Have you tried recording on other devices just to check?
I’m going to try another device over the weekend. I have a Nikon DSLR with video capability (but I think the microphone on that is pretty bad too). I was a little concerned that it was my fiddle–but I really tried to listen for a buzz and I couldn’t hear anything. I’ll experiment with what I have available and see if I can get something to work ‘well enough’ for now until I can figure out another setup.