Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.4.13

1.1.4.13

taddha smaitatpurā | jāyaiva haviṣkṛdupottiṣṭhati tadidamapyetarhi ya eva kaścopottiṣṭhati sa yatraiṣa haviṣkṛtamudvādayati tadeko dṛṣadupale samāhanti tadyadetāmatra vācam pratyudvādayanti

Now in former times it was no other than the wife (of the sacrificer) who rose at this (call, to act) as Havishkrit; therefore now also (she or) some one (priest) rises in answer to this call. And at the time when he (the Adhvaryu) calls the Havishkrit, one of the priests beats the two mill-stones. The reason why they produce this discordant noise, is this: