Tom is pleased by the sound of it (4)

I believe the answer is:

purr

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'sound of it' is the definition.
'purr' can be an answer for 'sound' (purring is a kind of sounding). I am unsure of the 'of it' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for purr that I've seen before include "Show pleasure" , "Sound of happy animal" , "Appreciative feline sound" , "Tiddles' sound" , "Express contentment" .)

I've seen this clue in The Globe and Mail.

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