go off: meaning and explanation
The phrasal verb ‘to go off’ is used to talk about alarms – alarm clocks, car alarms, fire alarms, and so on.
If an alarm ‘goes off’ then it starts, makes a sound or rings.
For example, I used to work with someone who slept so deeply that when his alarm went off in the mornings he would turn it off without actually waking up. This meant that he would oversleep and would often turn up late for work. Now he hides his alarm so that when it goes off he has to look for it and find it before he can turn it off. This gives him a chance to wake up properly.
Have a go at these micro-dictation exercises to hear this expression being used in context – how much can you understand?
About the sentence
…a car alarm was going off…
The continuous is used here to communicate the fact that the action happened over a period of time, instead of happening once or suddenly, e.g. a car alarm went off).