Hissing noise from pipe

Hi,

I have the following in the front of my house. It's now making a hissing noise but if I pull on it slightly it stops. Any advice how to fix it and what is causing it? I don't see any water gushing from anywhere.

https://preview.redd.it/l3cre8snyikg1.png?width=662&format=png&auto=webp&s=41cfd9ce93a39098e0d917d470aa3b28d3e89e6f