Say, you suddenly hear a sound,

What are the most popular question you want to ask?:

Do you hear the sound?; What makes the sound?; What is that sound?; What sound is it? What sounds?

According to Ngram, What is that sound? seems the popular one.

Say, you suddenly hear a sound,

Which is the most common question to ask?:

a) Do you hear the sound?
b) What makes the sound?
c) What is that sound?
d) What sound is it?
e) What sounds?

According to Ngram, What sounds and What is that sound? (dark green and light green lines respectively) seem to be the most common forms.