Hmmm, that's when things quiet down...you are likely to hear a lot of things. Have you tried taking a spin with the belts removed? Tried reproducing the sound in the driveway? I wouldn't rule out a discharge from the secondary wiring just because the engine runs well.

Someone posted a procedure for checking timing chain slack: Pop the cap off the dizzy, turn the crank back and forth, see how much the crank turns before the rotor turns. I don't remember what the acceptable limits are, all my junk is worn out. My chain sounded like a bicycle chain, my dizzy made several different and interesting sounds. This was while playing with a stethoscope in the driveway.