What is that clatter

Aside from valve train noise-which ticking is good-means valves are not to tight-Has anyone been able to isolate the clatter (almost piston slap noise) when the engine is between 1200-1800 rpms. Rule out main or rod bearings-that is not the noise-An Exhaust leak would not clatter on a 6 cyl engine-it would have a steady sound. It seems to emanate from the left lower drivers side-but not as far back as the bell housing

 

did you ever find any answers on this??? i have a valve/flapping noise coming from the same area. goes away when engine gets completely warm. it is especially bad on cold mornings.