Originally Posted by littledonny
watch out its going to blow ! ! !!!
In all seriousness, that's perfectly and completely normal behaviour. Dependant on processor brand, there are implements within the BIOS called SpeedStep and Cool'n'Quiet, these clock down the processor and lower the speed of the CPU fan to save energy when the CPU is idling.
When the CPU is stressed, the voltages to the CPU are increased as it requires, as such the CPU heats up and thus the fan speed is needed to increase to control the increased temperature.
Now, I'm uncertain if you were playing the same game beforehand, or what your previous Graphics Card was, however in all likelihood, the CPU was unable to use its full potential or was being held back by your previous Graphics Card (also referred to as bottlenecking), hence the reason you never heard the fan ramp up beforehand.
One way to check, is to open up Task Manager and play the game to where the fan speed increases, Alt+Tab and look at the CPU useage peaks, put in your old Graphics Card, rinse and repeat.
I am guessing that you're also running a stock HS on your processor, if there is any dust buildup you should get some compressed air to it and give it a good clean out, otherwise the only way to reduce the noise is by fitting an aftermarket cooler.
Anyway, you should be happy your processor is being used to its fullest, it's a good thing.
Assuming it's not due to a dust build-up and your airflow is being constricted.