Alright, "you never CEASE to AMAZE!" Sorry man, I've been burning the candle at both ends WAY too much lately and my brain is mush ( i.e I just accidentally insulted one of my favorite artists, possibly twice). So, I want to make it clear I ADORE your work, you're freakin' awesome. All apologies, I'll slink away now.
hmmm, tough question. sometimes in the tool, and sometimes it's how i use the tool for whatever needed effect. super clean speed lines are always nibs. the noisy linework can be multiple things. like drybrush, techpen scribbles, splatter, and white ink. it's really tough to explain. I'm sorry, man!
just know that the dirty stuff is almost always layered!