Ăn cơm nhà vác tù và hàng tổng

Literally: To eat your own rice but blow the village horn

Example: “Ông ấy ăn cơm nhà vác tù và hàng tổng, dọn rác cả khu phố mà không ai trả công.” — He cleans up the whole street at his own expense, and nobody thanks him for it.

Origin: The tù và is a horn made from a buffalo horn or a shell, blown to call village meetings, and the tổng was a canton of several villages. Eating at home but blowing the canton horn is doing public work for nothing, at one's own expense.