Rumbling during print - mystery

Ok, this is a first for me, and reaching out here for help - picked up a Creality S1 Pro on sale this week and noticing the following for all prints.When the extruder is moving back and forth sometimes there is a rumble - like a hollow sound. I believe it is the nozzle hitting the top layer, but not sure as I have increased the Z offset, which made things worse as the layers have gaps in between and layers don't stick. I took a bright flashlight to observe the printing, I don't see anything obvious when the noise occurs - it happens randomly - there is no pattern....sometimes the extruders moves and prints no noise, and out of the blue the rumble occurs. It almost is as if the nozzle just barely scrapes or hits a point on the layer. I have tried printing slower, raising the Z height, etc, but to no avail....if anyone can please chime in what this might be please let me know. Ok, but upon closer examination, I heard the nozzle make a click - like the sound of bending plastic, so I believe now that the nozzle is hitting something....I think it might be one of these filament strings as shown in the picture. What are these and how do I prevent them?Thanks

You could try using z hop if you think it is hitting something just turn it in your slicer.