Um, does the noise happen when you engine brake, or only when you have the throttle pressed down?
Cause if that's what you are talking about, then there has been some issues with this. My car does this, especially when the AC is on, but it's not the compressor, and no one knows for sure what it is.
All I know is that it isn't the engine, so I don't care.
Also, if it's when you have the AC on, then your compressor could be shot.