It is unlikely to be an "electrical" noise if it still occurs with the battery disconnected.
Find somewhere nice and quiet and slowly turn the wheel by hand and listen for noises.
There are many things that can cause unusual noises including the following:
- the tyre rubbing against the frame, mudguard, chain, panniers or motor cables etc.
- the rim rubbing against a brake block (can sound really bad if a piece of grit is embedded in the brake block)
- the disc rotor touching the pads
- a noisy or damaged wheel bearing
- motor harness touching the inner edge of the brake disc mounting (cooling fan) if the retaining circlip was not fitted correctly or has been dislodged
- the inside of the freewheel touching against a large diameter spacer washer
- the end of the freewheel (or sometimes the chain on the smallest sprocket) touching against the frame
- the derailleur touching against the motor side cover when the low gear is selected
- disk mounting bolts touching the speed controller if the bolts are too long
- surface corrosion on the stator making contact with the magnets inside the hub
- a loose wire rubbing against the motor side casing inside the motor
- worn wheel bearings with excessive play allowing the stator to make contact with the magnets inside the motor
There are obviously a lot more reasons for noises than the ones I've listed above, but at least you can start by ruling these out first.
If you check all the external possibilities first, you will hopefully find the cause without having to resort to removing and dismantling the wheel.
AlanP.S.
One more thing worth checking:
Make sure that all the spokes are all nice and tight as they can sometimes creak if they become loose. If the spokes are loose enough for the nipples to undo, the nipples can eventually fall off and start rolling about inside the hollow section of the rim which will make a strange noise with every revolution of the wheel.
