A/C problem 2001 Impala

First of all, this forum rocks, I have been able to gather some great info from here. I have a new problem though, when I turn on the AC, a fan on the back of the radiator starts making a very loud whining noise. If I turn the AC off, it stops shortly after. I live in Las Vegas, so no AC is a bad idea. Any ideas what this is? I am car-retarded basically. help!!!

the fan on the radiator is hooked to the A/c so that it is running whenever the A/C is on. It sounds like the fan motor may be crapping out but first check to see if there is any foreign object tangled with the fan blades.