engine noise

[grizz]

51 flathead 6 makes a dull knocking sound just before time to shift from low
to second and second to third. I thought it was a bearing noise so the crank
was removed and it only needed a light polishing. New bearings installed.
Its quiet at idle and moderate speeds and when pulling a hill. Has good
compression and starts right up with no smoke. Maybe someone has had
a similar problem? thanks