I dont know much about Puch other than you seem to have a primary transmission of some kind. It is not unusual to have a little backlash which will cause a knock at idle because the engine is speeding up and slowing down as it does the typical 2 cycle idle.
Try rocking the rear wheel back and forth while on the center stand and see if you can reproduce the noise to figure out exactly where it comes from. If it seems to be the transmission and goes away as soon as the engine smooths out, ignore it.
You might also check the front sprocket and make sure it is correctly attached.