I have a 2005 FX4 5.4L and only when the A/C is on does this sound occur. This sound occurs staring from a dead stop and goes away after shifting into 2nd. It is a loud whirring sound. Does anyone have any ideas what it might be? Thanks!

 

elaborate on WHIRRING

 

Are you running a stock or aftermarket stereo? Sometimes the electrical interferrence can create the sound that you are describing. But I may be way off.

Ox