knockinggerund / present participle Used attributively to describe something that produces or is characterized by a knocking sound.
Definitions
Producing a series of sharp, repeated sounds like those of striking; characterized by knocking. There was a knocking noise coming from the old radiator.
(of an internal-combustion engine) exhibiting combustion knock; functioning with an audible series of sharp bangs The knocking engine finally gave out on the steep hill.