Clouc clouc is not known as is in French but is likely from Occitan and not Basque.
In Occitan, cloc pronounced \kluk\ = clouc is the name of the sound made by a cloca, a broody hen. This sound is transcribed as cot cot cot codet/codec in French.
Its root can also be found in Spanish where cloquear and gallina clueca are both related to a broody hen. In French, the equivalent would be cocotte.
While clouc is undoubtedly an onomatopoeia like its English counterparts "cluck" and "cackle". Clouc might also be given a Latin root: the verb glocare which gave the French glousser.
Wikipedia suggests an indo-European root, *klag, so that might be a very ancient onomatopoeia.
References: cloc cloca clueca cloquear glocare clangare