So these past few days I have been 3D printing again with my Ender 3 with PETG and 0.4 mm nozzle and while I have been using the same setting as usual I am seeing an unusual amount of stringing between the prints. Does anyone know why? If you're wondering regarding the settings here they are, I'm using Cura for slicing, see options:
The travel speed of 160 mm/s is a big red flag. PETG is not tolerant of a hot nozzle moving over it at high speeds, especially unretracted (combing). The nozzle will drag material in a stuttering pattern, every so often, marring the surface and pulling what it dug up into strings.
Lower the travel speed to the same as the print speed, and then experiment with whether you can increase it without problems. I would not try going over 80 mm/s and probably not even over 60.
For what it's worth, this sounds like softened/molten PETG is a non-newtonian fluid, where at low stress (slow moving nozzle pushing/pulling) it deforms gracefully, but at high stress (fast moving nozzle) it strongly resists deformation and has a discontinuous breaking point. A quick Googling turned up this article, Thermal, Rheological and Mechanical Properties of PETG/rPETG Blends, which might explain what's happening.