It is a bit loud, I just don't see the point in keeping a constant 70 when the comp isn't doing much once I'm off Diablo?
Would keeping a constant 70% just be a good idea?
Also, will it affect the fan's lifespan, etc?
Use some compressed air to clean the GPU heatsink so the fan can effectively move heat away. Also, no brainer, try keep wires from blocking airflow and clean case fans. I wouldn't see 70% fan speed as a permanent solution!