Hi White . Thank you for your question!

　　Possible reasons are:

　　1. Excessive or insufficient grease;

　　2. Grease dirty;

　　3. Bearing damage.

　　Exclusion methods:

　　1. Check whether the grease is adequate. The grease should fill 50% of the capacity of the bearing seat;

　　2. Clean the bearing and replace the grease;

　　3. Replace the bearing.