Фонетические пары ʊ - u:

cooed - could 

kuːd  - kʊd

pool - pull

puːl - pʊl

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coo

/kuː/

verb

past tense: cooed; past participle: cooed

1. (of a pigeon or dove) make a soft murmuring sound.

"ringdoves cooed among the branches"

2. (of a person) speak in a soft gentle voice.

"she cooed with delight as he unpacked the bags"