why are my fans running full speed after hard drive update

My Western Digital 500GB hard drive crashed. The date on it is Aug. 2010. I swapped it out for a Western Digital Caviar Black 1TB. All plugs were identical including the temp sensor cable. My fans now run at full speed. What gives? BTW. This is th 3rd WD hard drive to fail at my office in multiple iMacs. I have a 21.5 iMac i3 3.06ghz