After a couple hours of riding my longboard it started making a high pitched squeak\squeal which sounded like it was coming from the brushless motor. After some investigation I found that a circlip on the end of my Turnigy Aerodrive SK3-6364-245kv Brushless motor was making the noise. The circlip usually rotates with the shaft of the motor but the motor bracket was stopping this from happening.
I used the DIYElectricSkateboard.com website “support chat” and spoke with Dexter the very helpful owner of DIY Electric Skateboards and he suggested trying washers on the 4 x bolts which attach the motor to the bracket.
I used 4 x M4 washers and the circlip now rotates freely which appears to have fixed the problem.