Rattle and Oils

Had the GTV out today, and its running great. But it does have one issue, there is a vibration/rattle. Something makes this irritating vibrating rattle sound when I take my foot off the throttle at about 4000RPM. I've been chasing this for a while. I isolated the hand throttle's cable with a rubber grommet and I tweaked the whole throttle linkage carefully to make sure there is no play. Now I'm thinking maybe its coming from something else... might have to in list some help finding this. Its a pain because it only happens when I'm rolling, but maybe I can find someone to look under the dash as I'm driving? Anyway, I picked up some oil for the car, Castrol Syntec 20W-50 "for Classic Cars". Has more zinc, which is supposedly important. Frightening how expensive oil is these days. I remember the old cans of oil being under $1 / quart...