estaban siseando
-they/you were hissing
Imperfecto progresivo para el sujeto ellos/ellas/ustedes del verbo sisear.

sisear

Mientras yo los escuchaba a ellos hablar, con mis ojos espirituales podía ver que las serpientes estaban siseando y hablando en su lugar.
As I see and hear them speak with my spiritual eyes, I could see that the snakes were hissing and speaking in their place.
Palabra del día
la guirnalda