At the opening of the poem, what annoys Grendel and leads to his attacks?
Within the first 370 lines of the epic poem Beowulf, one of the protagonists of Beowulf's is described in great detail. Grendel, a descendant of Cain, is awakened by the boisterous noise emanating from Hrothgar's mead hall, Heorot.
Given that Grendel is facing exile, deemed by God, for his relation to Cain, He considers himself a foe of God's. This being said, Grendel cannot enact his revenge on God and finds that the only way to hurt God is to take the lives of his followers.
Grendel decides to take out his revenge by murdering Hrothgar's people. Upon the first night of his revenge, Grendel takes the lives of thirty Danes. He continues to enact his revenge against the Danes at Heorot until Hrothgar decides to close the mead hall.
Basically, hearing the people of Heorot irritates Grendel given the fact he cannot approach the golden throne of God. Hrothgar built Heorot in order to praise God and the people at the hall are celebrating God's power. Based upon the fact that Grendel cannot enter into the light, because of his exile into darkness, he finds the only way to soothe his annoyance is to attack the mead hall.