I have a 2001 Mustang GT, when cold start occurs, and I simply let the engine warm up like normal, after doing so I put my car in drive ( yes its an auto, and when its put into drive, the rpm will change a bit ) so I get a loud droning noise from the front of the car, for about 10 seconds this noise will occur, and my rpms will rise a little over 1100, and then it will stall out completely, but then after i start it again, it will be a normal working car. I've checked my belt and all the accessories running from the belt, no sign of wearing at all. Ive had a mechanic look at it and they cant figure it out, I have a video of the noise to show you guys what I mean by it.