Answer:
Joe
Step-by-step explanation:
A participial phrase is a verbal phrase that acts as an adjective, modifying the noun in the sentence.
Hearing the rattle of the trees against the window, Joe jumped off the couch while watching a horror movie.
The participial phrase in the sentence is above "Hearing the rattle of the trees against the window".
The participial phrase above modifies the the noun "Joe"
It tells us something about "Joe", the subject of the sentence. It helps us understand why Joe jumped off the couch while watching a horror movie.