There are several possible reasons.

The first is the quality of gasoline. The label used is not up to standard and is easy to be knocked.

The second is that there is too much carbon in the engine combustion chamber, which will knock.

The third is that the ignition time is advanced, and it will also knock.

Mainly for these reasons, most likely.

Your car is only over 20,000 kilometers, so there shouldn't be much carbon deposit.

First of all, it is suggested to try to change the 97 # oil to see if there is still knocking. Generally, it is related to poor oil quality, or observe it at another gas station.

What's the road condition of your car at ordinary times? If the road conditions are not very good, it is normal to have carbon deposits. It is suggested to use some gasoline additives such as TCP.