The fan is running so loud.

Today I started my imac up and the fan is running so loud. I could put my hand in back of it and feel air blowing from it. Why this now, what's going on, can anyone tell me what should I do to get it running back to normal.

Also not long ago I up grade to a 2TB using the guide from ifixit which was awesome.

I searched loud fan on ifixit and it said something about the thermal sensors being detached, but it took a little pressure to put that on so I don't think it would come a loose that easy. So what should I do, Help please...