Definitions of clack:

• noun:   a sharp abrupt noise as if two objects hit together; may be repeated
• noun:   a simple valve with a hinge on one side; allows fluid to flow in only one direction
• verb:   make a rattling sound
• verb:   make a clucking sounds, characteristic of hens
• verb:   speak (about unimportant matters) rapidly and incessantly
• name:  A surname (rare: 1 in 100000 families; popularity rank in the U.S.: #8758)