Windows, an oil tank and the back door were damaged at the house on Main Street in Carrowdore, near Newtownards.
It happened at about 03:20 GMT on Wednesday.
Police said there were no reports of any injuries.
The occupants of the house are a couple in their 60s.
The house was cordoned off while officers investigated the circumstances and motive.
Police have appealed for information.
Police are investigating what caused a loud bang after a house was damaged in County Down.