You should warm up more thoroughly before getting into the actual workout. When it comes to pull-downs, i only feel my back muscles work if I do wide-grip pull-downs with overhand grip or if i do shoulder width pull-downs with underhand grip.
Underhand involves the Teres Major to a greater degree and definitely the bicep more than overhand or wide grip. Underhand lat pulldowns are a staple in my back routine. I like the vertical stretch it gives me and I can really contract hard on it.
When you say "cracking" I think of bone... is it more like a snapping sound? like a tendon maybe? Like Reinheart says, make sure you are thoroughly warmed up before your working sets. But sometimes certain people just can't do certain exercises... Like I get a cracking and popping whenever I do upright rows w/ a barbell...
Dorian Yates has said in MD magazine that underhand grip lat pulldowns are the most important back exercise and should be done first. To him, wide grip pulldowns work the teres minor more than lats. Have you seen the lat on Dorian?
I couldn't find a study on it. I would say consider preference first, it's important to enjoy the exercise I think, and variation considered as well.
To be honest I haven't use the triangle attachment in a long damn time. The rubbing between my arms and lats gets irritating and distracts my focus on feeling the contraction. I'll opt for the cambered attachment for a neutral grip lat pulldown and cable row.