Knocking sound made by Rex thrice an empty act (3-1-3)

I believe the answer is:

rat-a-tat

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'knocking sound' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for rat-a-tat that I've seen before include "Knocking sound" , "Rapping sound" , "Noise of knocking" .)

I've seen this clue in The Hindu.

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