Mr.O, You are right. They mod their bike which they steal from someone. They are trying to rev motor like music, but usually you don't want to hear cause its super LOUD!!!
They usually ride in the midnight near residential area, so COPs often chase and arrest them. This picture is from a parking space near police station where they keep captured bikes.
This stupid culture has never ends from 30 years ago. It is really annoyng for normal people.