I've got some kind of noise and I don't think it is pre-ignition. Another friend thinks it is just the sound of the exhaust at that specific rpm because it goes away right away when I get into the throttle a bit more. The engine builder also suggested it could be the sound of the exhaust. I don't know though.

I swapped out for one range colder plug (the AutoLite version of the RC9YC4). That didn't make a bit of difference. I put the medium spring where the one with the slot used to be. Now my timing is all in by 3100 rpm (used to be 2400 IIRC) and it still makes the exact same sound at the exact same speed. I also tried backing the timing down again from 16/34 to 14/32 and it didn't make any difference other than the engine wanted to get a little warm in traffic.

Its gotta be something else. Right?

And I generally run 93 from Shell or something similar.