Any MB gasoline engine as far back as the mid eighties has a knock sensor which supplies information to the engine management computer which will retard timing to prevent spark knock. If the knock was just for a second then the system is working correctly and retarded the timing properly. If the knock was continuous, there is something wrong with the system.
If there is something wrong with the system causing the knock to be continuous, it can damage piston tops. In the old days of high compression engines, no engine management computer with knock sensor and low octane fuel, spark knock would put a hole right through the top of a piston.
Every time you here that knock it is the equivalent of a LARGE ball peen hammer hitting the piston top. The force is applied quickly and to a very concentrated area of the piston top.
I would try to ascertain if the knock only lasted a second or so and then went away. If so, you probably have nothing to worry about.