I've been riding my Typhoon the last few weeks and have noticed that occasionally usually on the overrun when I've stopped pedalling that there's a slight shiver of a vibration through the bike, only for a second but noticeable. Was wondering if that was normal? The bike appears to run fine other than that and powers to 15.5mph (at which pointing flying along turns to cycling through custard as the motor switches off) quite happily plus the wheels seem to spin quite freely.