I think this to be the tremendous roar of the wielder of the thunder, viz., Vasava, who cnraged at the slaughter of Drona is coming to fight on behalf of the Kauravas.
सेन्द्रानप्येष लोकांस्रीन् ग्रसेदिति मतिर्मम। मन्ये वज्रधरस्यैष निनदो भैरवस्वनः॥