Loud engine noise durring acceleration and free rev

My dad has a 2002 Chevy Impala with the 3.4L engine and has 66100. When we start the engine it will idle fine but once we try to accelerate it makes a loud noise coming from the tube going to the air filter. We have changed the fuel filter,plugs, wires, air filter. I took it to the dealership and they said that it coul've been the catylitic converter so i changed that too and no help. Now the chevy dealership has no clue what is wrong the engine isn't giving out any codes to help diagnose. PLEASE HELP!!!