France's Warning Sirens
No, it isn't to announce the Corona virus pandemic. The alert system which is tested every month is part of the SAIP (Système d’Alerte et d’Information des Populations), a national warning system designed to alert the public if it is in danger.
The siren "can warn the population in a given area of imminent danger," says the website of the interior ministry, adding that "this network, inherited from the WWII was initially designed to alert people of an air threat (conventional bombing or nuclear) and can also be used to cope with the rise of technological or natural risks".
While it can come as quite a shock if you're not used to them, the monthly test ensures the system is still working and you'll see them dotted around, particularly on the roofs of municipal buildings.
In fact, some of the oldest are located on the roofs of churches and aren't automatic which means that on the first Wednesday of the month you could spot a municipal employee scrambling up to push the test button manually.