My quad (U818A) recently started having one of the arms spin all by its self. As soon as I plug in the battery the propeller starts spinning. It is not spinning at full power but still pretty fast. This happens regardless of whether the remote is turned on or off. additional info; I recently broke that arm and had to replace it. I've flowing it a couple of times prior with no issues. I smelled a burning smell right before it started happening. It is the rear right arm and I've tried another brand new arm with the same result.