To yammer is to make a loud, howling sound, or to talk on and on about nothing much at all. When cats yammer outside your window all night long, it's hard to fall asleep.
You might complain about the way your neighbors yammer loudly in their yard early in the morning, or about the fact that your brothers yammer and fight all through dinner. Whether a person, an animal, or an inanimate object like a motorcycle yammers, it makes a pretty annoying sound. In the fifteenth century, yammer meant "to lament," from the Old English root geomor, "sorrowful."