the lull before the storm
From Longman Dictionary of Contemporary Englishthe lull before the stormthe lull before the stormPAUSECALMa short period of time when things are calm, that is followed by a lot of activity, noise, or trouble → lullExamples from the Corpusthe lull before the storm• Then came the lull before the storm.• There was a feeling that something was in the air, that it was the lull before the storm.