This tip is new to me and a lot cheaper than buying Space Balls for solid wood panels...
"Solid wood panels expand and contract seasonally. When they're at their smallest they can rattle within the frame. You can take the rattle out with these silicone strips. Make them by squeezing 1/8" dia. beads of silicone caulk onto a piece of wax paper. After the caulk dries, peel off the strips and cut them into 1" lengths. Set these into the grooves before you assemble the door."