I had this same sound when I got mine. STOP riding, you need to either regrease the main motor bearings by taking the shields off and adding more or cleaning it out and adding more. You can remove the back clip off with pliers if you can’t fork the back shield off.
Maybe check the wheel adapter for scraping in case the smaller motor bearing has been scraping it before assuming it’s bearings. This sound though completely resembles bearings that have spun too fast with too little lubrication.
Look at videos on how to remove bearing shields and you need to repack the bearings. Mine did this exact shit but now are amazingly smooth. Buy fully synthetic grease from your hardware store.
Also triple check your spacing and if you need to add speed washers on the Inside of the wheel.
You have to do both bearings, the big one pictured in my post and this small one under the wheel adapter. I don’t know which one is the problematic one but both had similar grease jobs:
Also if you’ve already ridden miles with this noise, have a good look where you maybe have excessive wear which may require a good cleaning to spot. You might still be ok but find the source of the noise before riding more. If you’re not comfortable with this or don’t know anyone knowledgeable you should talk to TB support.
