What's That Sound?

I just got back from 3+ weeks in sunny Huntington Beach and the whole time I was gone this issue was on my mind. I got a cheap stethoscope from Amazon and when I got home today fired up the car and started sticking it where I thought it was coming from. Nothing in the valve train that I can hear. I stuck it to the manifold, nothing. Heads, nothing. But I can hear it...so I stuck it to the distributor and bingo! I pulled off the cap and see dust from the rotor scraping the cap. The DUI dizzy I have doesn't fit very well under the wiper motor and was slightly askew. I had a new cap and rotor so I put them on, removed the wiper motor and had to dimple the firewall a tick to get one plug wire to fit properly but it solved the problem completely. I can't figure out why the knock/tick/tap didn't stop at some point with wear, or why it just started out of the blue, but it is gone.