I had found in the past that stringing would often go away with the change of hotends. With the last nozzles I purchased came .4mm needles. To my surprise about 1/3 of my nozzles were small and the needles would not pass through. I then did a filament load on a machine that was stringing. The filament did not flow out thick and heavy. I took the nozzle off and sure enough I could not pass the needle through it. I replaced the nozzle with one that the needle passe through. The filament load looked great with a nice heavy flow. In addition the stringing was gone. I posit that you get the same output with a smaller diameter nozzle because the extruder can exert enough pressure to force out the same amount of filament. The problem is that the pressure inside the nozzle is high and during travel releases as stringing. I now periodically check the nozzle by doing a filament load and seeing if the flow "looks" right.