My grind/rattle/flutter started exactly the same way. Very subtle and very infrequently. Overtime, over approximately 30 miles of riding, it has become more consistent and occurring more frequently. I can reproduce it now reliably by peddling just below the speed that the motor would theoretically be operating at. This can be done by coasting, , ( When the motor is not activated by any torque or strain through the crank ) and gradually increasing your pedal speed just enough to not produce any torque on the crank and it will make the sound reliably as long as you don’t put any torque on the crank which would turn the motor on. Actually it will be reproduced now by applying slight torque On the crank , but not enough to really get the motor to engage fully . This does not happen when the motor is turned completely off . ( ie When the control is powered off completely, or when the control is on but a long press of the lower speed, stops even eco )