Squeeky and rattle sound when A/C is on

First off let me say that my car has been to the dealer at least 4 times for this. They claim they can hear or find nothing. This started about three years ago.

When a/c is on, we get a sqeeky/rattle sound coming from the engine compartment during normal driving, when deaccellerating, when stopped at a stop light, or when applying the brakes to slow down or stop. It does not make this noise constantly while driving around. To me it sound like a loose tensioner or something similar. I've suggested this but have been told no.

This problem cannot be duplicated on the vehicle when it's just parked. I've tried. I start it up, turn on the a/c, apply the brake, put it in drive, and hit the gas pedal. Nothing! I've also had the vehicle idling in park with a/c on and just revving up the engine; still nothing. It only makes the noise while driving and only when the a/c is on.

I'm taking the car back to shop one more time in the next few weeks for one last check by them before my extended warranty expires (about 800 miles left on it). I'm afraid the warranty will expire, part(s) will finally fail, and I'll be stuck with a sizeable repair bill.

Any help would be greatly appreciated!

Respectfully yours,

Mark Reynolds