That the effects of noise pollution are hard to formally describe (and extra hard to quantify in monetary terms) does not mean we can just throw up our hands and pretend they don't exist. In fact, I'd say this is exactly why we stake out lofty non-economic ideals like human rights etc - markets have friction (ie are not efficient), and this combined with your reasoning means we could justify anything that produces negative outcomes for humans by simply writing it off as non-economically-legible. But the entire human-value justification for letting markets do their thing rests on the positive benefits they bring to us humans, so that is absurd. And so we always need to look at the larger picture and judge the real-world effects on humans.
But that's pretty much the definition of the term. It's a negative impact on a third party. If I'm living next to a data center, and it's noisy, or my electricity costs more, those are negative externalities.
There are real reasons not to want data centers next door, unlike 5G towers.