i just changed my 2005 Sienna AC compressor 2 months ago. Now I have noticed a loud sound when ever I turn my AC on. It gets louder when I slow down or on idle .

The sound is not shrieking but it's like a louder engine-like sound or motor-like, like a loud hum. No vibration noted. I hear when AC is on, especially when I am slowing down or at idle ( maybe because I hear it better then?). But noise is gone when AC off. One mechanic said, I might have a defective compressor. Any ideas?