How do I make this old computer run quietly?

I have an old HP Pavilion XE736. I would like to set it up as a Linux-based hardware firewall. That I can do.

The problem, it runs so loud compared to my other computers that it's annoying and ends up being a bother, so I skip it.

What are steps I can take to make this (or if that's too specific, any) old computer run significantly more quietly?

D.