We don't know! Because Fliiiight is so quiet, occasionally users discover rubbing brakes, an out-of-true wheel, or out-of-tune drivetrain parts they can't otherwise hear. In most cases it's nothing serious, but check with your local bike mechanic just in case. Additional information, such as whether it is happening once per wheel revolution or once per crank revolution will help them diagnose the issue.
Note that Fliiiight's motors do make some noise, which is audible on start-up, but in many cases is not audible over your drive train once the wheel is spinning (see also: How quiet is Fliiiight?).
If the magnets make contact with the wheel, resulting in noise see: Fliiiight's magnets make contact with the wheel