That noise is more or less normal, however it can break-in over time especially with some hill climbing we have found.
The Bafang(8FUN) BPM is made for 36V sets up originally and larger diameter wheels. Its a 10 year old design and reliable.
1. We run it at 48V so that you can go faster, and have great torque and high value. The motor over time has been improved to run at the 48V.
The result is the motor is spinning quite fast. It is within spec, but the extra speed make the gears clatter around due to some harmonics.
Something like when the something in the dash panel of a car rattles at certain motor RPM. Its kind of a weakness of the current generation of BPM.
At lower RPM the motor is quite quiet, but when its higher or going downhill it makes more noise.
2. The materials of the BPM have been improved (harder more durable) but the noise level is higher lately.
3. We will see this more often in the 32Ah bike as it goes significantly faster than the other capacities due to the stronger cells run the 32 Ah pack.
4. In the ODK U500 the motor is in front. So the motor noise will be right under you and will hear it more than if it was behind you.
5.Check your bike accessories. if something is lose, it will vibrate at certain frequencies and make some noise.