Giant Quick E - cracking sound

Video of cracking noise from my Giant Quick-E -1917. This problem has been examined by the Giant dealer. The last solution tried was to change the motor (the bike is still on guaranty). The problem was even worse (video was made at that time). The cracking comes and goes, it can be there for 200km and than stop for 100km. It is a real enigma. If anyone can help please leave a comment. Thank you.