It's because small sprocket is between ~12 teeth and rear one ~52 teeth. Front small sprocket spins quickly and the chain is smashing at high rpm at front sprocket teeth, therefore the sound. You can decrease sound increasing front sprocket teeth by few teeth and decrease rear sprocket teeth or leave rear sprocket as it is, change the ratio little bit, but it might have a negative effect on torque and efficiency and speed.
These motors were used in motorcycles with 48v or 52v or 60v don't remember exactly and where doing 100km/h with modest amperage. These motorcycles had ~ 4,8:1 ratio. This motor is quite slow and it is good in a matter if you want to have less sound in chain rear transmission.
eq. if front sprocket would 25teeth and rear 50teeth than it would be quiet, like with hub motor where it is used as mid-drive motor like in vector or new quilbix frames.
Another option to use pulleys and belt instead of the chain but then again propper belt tension becomes problematic and technically challenging.