I just replaced the optical drive in my mid 2007 imac and when it powered back on the fan inside is running at max speed, it sounds like it is getting ready to take off!! I replaced the hard drive a few months back and didn't have any problem at all. I replaced the temp sensor on the optical drive in the original location. Resetting the SMC controller doesn't help, also software fan controllers do nothing, what now? Is there something that I need to go back in & make sure is connected?