Had same problem,
Heard the same fender suggestion. Sent them a recording. My free warranty replacement wheel is arriving today. Thanks Lectric!I have a step-thru 2.0 and an XP 2.0. On my XP when going approximately 21-22mph and over (although only goes to 24mph) I get a grinding sound coming from the rear hub. This is on a flat surface, with no headwind. I weight about 165 lbs. I made a video and posted it on YouTube. I sent the link to tech support but they suggested it might be a loose fender. The sound immediately stops when letting off the throttle, so I really don't think it's the fender. I could swap controllers, but I'm saving that as a last resort. Here's a link to the video. The sound start at about the :17 sec mark: