Many reasons cause this, some may include meshing of the gears might be wrong, broken bearing in motor(yes, that does happen), gear/differential scraping parts of the car.

• 很多可能性:可能是齿隙造得不好(扫齿)；轴承坏了；齿轮/差速摩到壳/车身；差速太松；齿差速坏齿；车壳和车身接合太松等...