The surface is pitted and inconsistent, and the nozzle audibly crackles or pops during printing.
cracklingbubblysteamhissing while printing
Causes, most likely first
Ordered by how often each is actually responsible, not by how interesting it is. Work down the list.
Wet filament — this is essentially diagnostic
Moisture flashing to steam at the nozzle. Dry the spool at the material drying temperature. Nylon needs drying before every print and feeding from the dryer.
Filament left out of a sealed bag
Store spools with desiccant. Nylon absorbs enough moisture from room air in a few hours to ruin a print.