If it's not running funny at all speeds, it's normal. At slow speeds under heavy load, you actually hear the vibration casued as the current shifts the pull from one magnet to the other. It gets louder the more load the motor is pulling. It gets louder if you pump up the wattage too.
I think of it as the sound of making heat in a motor. So to the extent you can, you can ride more efficient if you ride so you hear that noise less.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.