Hi and
to the forum.
From your description, you probably have one of the small geared mini hubmotors:
These motors have an internal unidirectional clutch mechanism
(freewheel device) which only allows torque from the motor to be transmitted in one direction.
This means that the wheel
(and hub casing) will manually rotate in the forwards direction
without actually turning the motor itself.
As the noise only occurs when the motor is running
(or being turned in the reverse direction) it will either be something catching between the fixed stator and the moving magnets/drum which revolve around it:
Or possibly a fault within one of the two bearings on the axle shaft in the centre of the magnet drum which can be seen in this drawing:
You can find out more about what is inside the mini motors in
this thread.
Unfortunately, you will need to dismantle the motor to try and see what is causing the problem.
If there is no sign of anything physically rubbing against the magnets or stator assembly, it may well be a noisey bearing in the centre of the magnet drum that is causing the noise.
Alan