Noisy Hangkai

Noisy high pitch whirring sound from the Hangkai bought from Aliexpress seller Shanghai Taiyou Industry Company’s store Order : 60292835208986. Any knows what is the problem ? I have changed the gearcase oil at the properller. But the sound persist. If you notice the video, there is already some rust forming at the crank chord cover at the 2nd try in the sea. I have washed it with fresh water after the two use. The motor start up easily with neutral gear before you throttle it. The forward gear