Tin Pest, Gram and Mockingbird found the source of weather anomaly (namely - a summertime amidst nuclear/megaspell winter), took it with them, and ran into ghostly horses... One of which let out blood-chilling scream at the party. Mockingbird hid behind Gram, Tin tried to burrow into the melting snow (the anomaly follows its source, after all...), and Gram... Gram's hearing, unlike normal diamond dogs', is impaired due to extensive work in the mines in his younger years. So he was totally not impressed by the ear-ripping screeching ghostly horse made.