'40 flathead engine noise

[Brooks]

My v8 flathead engine is making a pecking noise when it get hot. was rebuild about 200miles ago sit for 9 mos. while I restored my car. the motor is a stock 1940 v8. run good until it get hot driving it then running about 50 mph making the pecking noise. No noise at idle. Anyone had this problem or know what it could be. Also has good vacuum 21 at idle and all plugs are firing good. Have a Blessed Day!