[singular, uncountable] a situation in which you are in a hurry and need to do things quickly
I can't stop—I'm in a rush.
What's the rush?
‘I'll let you have the book back tomorrow.’ ‘There's no rush.’
The words came out in a rush.
http://www.oxfordlearnersdictionaries.com/definition/english/rush_2Here "rush" is classed as uncountable but still in most example we could see the "a" before rush. Can you put number before rush that is how I identify if the word is countable or uncountable?