I ran a 12t for a while. That noise is the chain on the the plastic guide-thing just before the counter-sprocket. I've since put a 45 rear and ideally would switch between a 13 and 14 counter... I use a 104 link chain to accommodate the larger rear sprocket.
I've found that the 13/45 combo is versatile enough to stay on all at the time. It crawls in first gear, and tops out at 75 in sixth--and due to the higher revolutions, 6th gear is less prone to wind and incline dampening.
The 13/45 combo is pretty close to the 12/39 you are running.
The future is wide open.