CPU Speed high?

[ukgrad89]

I recently replace a dead hard drive in my iMac and since then the fan noise has been loud.
(can't post my graph) But the little red circle has the cpu fan speed around 3000, but on the sloped red line given the temp should be around 1300.

Isn't the red circle (CPU fan desired speed) suppose to be closer to the sloped red line?
What could be causing this issue?
I have double checked the HD temp sensor (it was a little lose). And I have seen the temps all go to -128. But that seems to have stopped.

Any ideas?