Engine knock at low low RPM
 

Anyone ever had their engine knock at low RPM? I mean very low, just about to stall out RPM.

Ive had it happen a few times when not paying attention to letting the clutch out and not giving enough gas, and it knocks a bit and sputters untill it gets moving..

Had it happen again today putting some more seafoam in, let the idle drop too low and it knocked a bit trying to struggle to get back up to RPM..

What is this caused by...