General Synonyms:
* Complaining
* Grumbling
* Whining
* Petulant
* Cantankerous
* Cranky
* Irritable
More Formal Synonyms:
* Captious
* Carping
* Fretful
* Peevish
* Testy
* Morose
Synonyms Emphasizing Dissatisfaction:
* Disgruntled
* Unhappy
* Discontented
The best synonym to use depends on the specific context and the tone you want to convey.