2002 Exp. horrible, loud rattling/chirping sound!!!

[Lady Edge]

I have a REALLY loud vibrating/rattling/chirping noise coming from my Expedition. Definitely from the front end, more towards driver side. Seems to be worse if i've been driving a while. (Drove it to the gas station tonight after it sat for about 6 hours and NO noise) Sometimes gets worse w/ AC on, but not always. Noisehappens in any gear AND in park but NOT in neutral..Usually waits until I am at a drive through window or near a friend to let out it's horribly embarassing shrill noises..ugh! Any suggestions as to the cause? If so, how would I go about diagnosing it and fixing it? Details please. I'm a young female. I can fix anything, I just have to know what it is and have a little guidance. Any help is appreciated.

[TBIRD430]

When was the serp belt replaced last?

It could be the a/c compressor clutch (it it only does it with air on).
It could be a failing serp belt tensioner too.