engine knock

I need help, run out of ideas. Engine starts to knock suddenly. It's knock at idle below 2000-2500rpm. Sounds really bad and I've got no idea what is this. Was replaced the piston because of slack and clutch basket buching (it has grooved a little). Was checked connecting rod and crankshaft its...

Just picked up a DRZ400sm. I noticed a knock in the engine other day in traffic when I revved it up from idle with the clutch fully disengaged. It doesn't happen every time I do it, but when it does, the bike kicks out a white puff of smoke. Ideas?