I know this has may have been already ask and answered but here it goes.
Should I put felt paper over or under the underlayment for 3/4 hardwood nail down.
I am going to use 3/4 cdx plywood over diagonal plank subfloor.

thanks

Both, it is not only there to resist moisture, but to reduce wood to wood squeaks.

Long ago I worked for an old timer, and here in the land of concrete. Wood screeds were used. Hot mopped asphalt tar and tar paper. Then plywood. He would spread out newspaper and start nailing wood. It was only to reduce squeaks.