Chirping noise in 2002 Ford Explorer

I have a 2002 Ford Explorer. I purchased it about 2 years ago. There is a chirping noise when it is running. Not only when it is driving but even when it first turns on. It started off faint but now it is really noticable. If anyone knows what i can do to fix it or knows what is wrong please let me know.

tighten or replace the serpentine valve

what's a serpentine valve? Have the same problem they replaced belt adjuster, but it started back in a couple days. I'm stationed overseas so the service it's that great, they told me to come back after holidays to replace adjuster againgo figure huh.