I don't believe anyone knows 100%, exactly why they howl & when they howl. As well as why don't they howl. Not to mention what each howl/yip/bark "really" means. A big discussion of opinions, for sure.
Perhaps they moved away, or were just passing through{tresspassers}. Or were just gathering the pack. As far as train whistles, sirens & some other loud noises, ie; calls. They sometimes seem to respond & sometimes not.
Personally, I've stalked in on quite a few in daytime winter snow. Using, a train's whistle{in the background}. To help mute my walking on loud snow. Never seen one howl or bark from those whistles, during those hunting jaunts....go figure /ubbthreads/images/graemlins/confused1.gif.