Why does the loud speaker hiss when in messaging?

[cined1]

When I go into messaging on my 4S and put my ear close to the bottom speaker, there is a hissing noise - The same noise you get when you put your ear into a shell.

My sisters iPhone 4 also does this.

Why does it happen?

 

[Vansslipon]

Wirelessly posted

Hehe no idea ... Nice found. My iPhone4 also has it )