Check the undersides of the rocker arms where they contact the valve stems. Quite often, they will get cupped to the point a feeler gauge doesn't give an accurate adjustment, and until the engine gets fully warmed up and the valve stems and pushrods "grow" a little the clearance is just a little too wide, causing a rattle. Some oldtime machinists (myself included) have an accessory for a valve grinding machine that resurfaces the valve stem contact area of rocker arms and eliminates the adjustment error.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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