From NPR:
The loud noises you may hear blasting from your electronic devices this afternoon are no cause for concern.
At 2:20 p.m. ET [1:20 Central] on Wednesday, the federal government will be testing two emergency alert systems on televisions, radios and certain cell phones across the country.
https://www.npr.org/2021/08/11/1026679111/fema-fcc-nationwide-emergency-alert-test-eas-wea