Imaging you're sitting in the hall and the truck comes up the hill, you note "hearing, hearing" and you go back to the meditation subject. Fine, that's what we teach. You actually heard a sound but you realized what was going on, there is just "hearing, hearing", and you immediately came back. Now, in the process of hearing, you may or may not have perceived what that sound was coming from because it really doesn't matter what made the sound. If you actually perceived it as a car/truck sound, that can be fine, too, but we still realize that we are hearing, we note "hearing, hearing", and we come back. This chain does not continue into mental formations.
However, imagine we're sitting there, the truck comes up the hill, we hear the sound, we perceive, "Ah truck!" and then we start building mental formations on that perception, "Why are they allowing that truck to come up the hill? This place is supposed to be quiet here. They should not allow the truck - that yucky truck - I'm going to tell them off when I get out of here." And all of a sudden the mental formations have gone into anger, into aversions at the truck, towards the teachers who have allowed the truck to come up the hill, and so on. That's when the mental formations are taking over, and that invariably always end up in Dukkha.
You may remember Rosemary's story about seeing the nice trees and all of a sudden she's lost in India, lost in relationship Dukkha, and so on. That's what mental formations can do. If we can stop at the perceptions by noting just "seeing, seeing", "listening, listening", whatever, then we've turned back to the meditation subject and at that moment we've stopped Dukkha. That's how valuable, that's how super-valuable mental noting is.
Imagine you're lying in bed and somebody's snoring. You know, it's like stress, all of a sudden there's this, "Ughh, they should shut up, they're bothering me". Ok, what happened? We heard a sound. If we note "hearing, hearing", it's easier to go back to sleep. The moment we have labeled "snoring" we're labeling what that person is doing, not what we're doing, and then the snoring brings about the issue of them, and then we're going to build a story around them. So if you don't feel your mental noting is good enough, get it going better, it's going to stop so much of your mental Dukkha, it really will.