KLX 250 loud clunk from engine
 

I have a 2006 KLX 250 (D-Tracker). Sometimes, after riding for about 30 - 40mins, stop and start the engine again after few minutes there's loud clunking sound coming from the engine. Noise only happens once when the engine is turning over. No issues while running. Also it doesn't happen when the engine is cold. Could anyone please help me figure out what the issue is? TIA