Groan
Part of speech: noun
Definition: A low, guttural sound of frustration.
Part of speech: verb
Definition: To make a groan.
Moan
Part of speech: noun
Definition: a low, mournful cry of pain, sorrow or pleasure
Part of speech: verb
Definition: to make a moan or similar soundto complain
Example sentence: Scientists habitually moan that the public doesn't understand them. But they complain too much: public ignorance isn't peculiar to science. It's sad if some citizens can't tell a proton from a protein. But it's equally sad if they're ignorant of their nation's history, can't speak a second language, or can't find Venezuela or Syria on a map.