A heat exchanger assembly including a plurality of pairs of plates disposed in series for fluid flow from a pass through one pair of plates to a pass through the next pair of plates. Each pair of plates includes a central rib to define a U-shaped passage having a fluid entering leg and a fluid exiting leg interconnected by an open bottom interconnecting the legs below the lower end of the engaging central ribs. A plurality of dimples project into the passage to interact with fluid flow through the passage and each of the dimples has a hemispherical shape and a divider rib is disposed in the passage to co-act with the hemispherical dimples to reduce whistling noise.