Hard Drive Fans slowly increasing speed

Hello everyone, in short I have two 21.5 inch iMacs, one from 2010 and a new one from mid 2011. I damaged the old one and bought the new one. The old one has a 1TB HD and the new 500GB. The old one receive a Brand new OEM HDD from Apple thanks to the hard drive replacement program. I tried to swap hard drives and they worked fine until about 30 40 minutes when the hard drive fan started to spin really fast and noisy of course. I did an SMC PRAM restart, repaired permissions, updated OSX and the problem keept going.

I read in some other place about the issue with Apple´s propietary temperature sensor but all I have read is that the fans start to spin really fast as soon as one turn on the computer and in my case it takes some time. I even installed SMCFancontrol to reduce the speed but there is no change, the speed remains the same.

So my question is do you guys and gals think this is releated to the Appe propietary temperature sensor or something else_ maybe a faulty HDD (although SMART says is OK).

Is funny because both are Apple OEM Seagate basically the same model. And the 1TB one is Brand new, one might think that if Apple is pushing it´s temperature sensor all the new HDD should have the same technology.