Could be a coolant bubble or something more sinister like a blown head gasket. Have you noticed the coolant level decreasing?
Our Father, who art in Autos, hollowed be thy frame. Thy Horsepower come, Thy torque be fun, in Imports as in Domestics. Give us this day our daily fuel, and forgive us our hooning, as we forgive those who hoon against us, and lead us not into chrome accents, but deliver us from rice. For thine is the Cam, and the Piston, and the Rod, for ever and ever. Or until they break.. Amen.