Noise coming from passenger front side of car when accelerating with AC or Heater on

While driving my Corolla with the AC or the heater on when I accelerate I hear a rattling noise coming from the front passenger side under the hood. I took the car to the dealership twice and the first time they said it was not heard by their mechanic after putting the vehicle through a diagnostic test. A week later I took it back and they said they could find nothing wrong with it but did tighten up a few things around the motor. When I left it was still making the noise and I had the service manager get in the car and I drove it so he could hear the sound. He said it was my muffler going bad which sounded like BS to me. It only makes the rattling sound when I accelerate and the ac or the heater is on. Any answers for this old woman who thinks she was taken for a ride by the dealership.