hi if its sounding like glass the embryo died and you didnt notice and still kept incubating it till eventually it makes that sound and its about to explod, throw it away
listen all my birds ive ever hatched and put them in the brooder theyve all died its onnly recently that i know the incubator dosent kill them now it is on week 1 so if this happens to you get an incubator with a fan and put it 36.1