Diagnosis
Normal motor noise. The E300's brushed DC motor naturally produces an electrical whine that varies with speed. This is typical, not a fault.
How to Fix It
No fix needed for normal operational noise. However, you can slightly dampen the perceived sound by ensuring all body panels and the deck are screwed down tightly, as loose parts can vibrate and amplify the sound.
If That Didn't Work
If the whine suddenly changes to a screech or is accompanied by a loss of power, it could indicate a failing motor bearing or severe brush wear.
Heads Up
A new, sudden, or dramatically louder whine paired with a burning smell means stop riding immediately—the motor is failing.