[JamesMA]

Check out the Limp mode threads. Basically the harsh fan noise is a result of the car thinking that it is overheating, so to "protect" the engine, the fan comes on to cool things and you will notice that if you step on the pedal hard, you do not get the a creation you are used to.

If it happens enough, that will eventually throw an error code ande the dealer will be abe to fix it.