If you do the CPU replacement and afterwards the optical drive fan and cpu fan are running at max speed (very loud) then you more than likely didn't plug in the CPU temp sensor. The guide here doesn't talk about it so you can easily forget it! The connector is on the other side of the board, which is different from all the other temperature connectors and not on the same side as the heatsink screws either.
I did this and the computer ran with noisy fans. I used a fan control app to quiet it down, but it was still loud on startup until login. More importantly, the performance without the CPU temp sensor connected was terrible. It was slower than the original i5 CPU. HD videos would stutter and benchmarks gave very low scores.
Make sure you plug in the CPU temperature sensor after you put the heatsink on.