Noticed today while on my road trip that there was chirping going on under the hood. Had no idea what it was, it has a new serpentine belt.
Then noticed when I turned the a/c off the sound went away. Here's there thing it didn't turn on and off like it would do if it were the clutch, but once I turned the a/c off the chirping would slow down until was fully off. (Does that make sense?).

For example...

AC on:
chirp-chirp-chirp-chirp-chirp-chirp

Turn AC off:
chirp-chirp--chirp---chirp----chirp-----chirp------chirp--------

 

It might be the belt tensioner pulley if it hasn't been replaced. I had a noise that was more of a grumbling when the AC was on, that ended up being the tensioner pulley. My RL was probably 112000 at the time. However it did not go away in a gradual way, as your noise does. Id probably just go and listen and try to pin down exactly where its coming from.