"Ви чули грім?"

Translation: Did you hear thunder?

[zickovski]

Did you hear A thunder?

[mag_svyatoslav]

Have you heard A thunder? - не прийняло!

http://context.reverso.net/
I can distinguish a cannon from a thunder. = [Я можу відрізнити гармату від грому.] - чому тут "a thunder"?
I thought I heard thunder last night. = [Я думав я чув грім учора увечері.] - а тут артіклю немає.

[shlyovich]

'A thunder' is grammatically incorrect. Thunder is a mass noun, similar to rain or water, and cannot be counted, hence we don't use the singular 'a thunder'. You can have an instance of thunder or a blast of thunder if you want to refer to a singular event.