Just to throw a curved ball here... If you are hearing the fan on "start up" from cold I don't recon it is the cooling fan. I recon you are hearing the "Air injection" it sounds like a hairdryer motor running flat out and from time to time it will come on for short periods. When I first heard this I wondered what it was too.

If you have an SRT-6 try this. Start the car, from cold. Quickly open the Bonnet (hood) and on the inner wing in front of the Battery there is a black drum like device. This will be making the noise you are on about, I recon. Apparently it is totally normal, but don't ask me what exactly it does.