Why dogs howl at loud noises

It all comes down to their family history, more particularly their wolf roots. 

Wolves howl whenever a member in their pack is harmed or in danger, providing others a signal of their location.

So instead of the common thought that a dog is in distress by the sound, you can instead interpret it as them signalling their location. 

Your pet dog is simply mistaking an emergency signal for a buddy in distress.

But while the behaviour is considered normal, it can sometimes signal separation issues from an owner.  If they continually howl without loud sirens perhaps try to increase activities with them or asses the issue with a local vet.