In this work the gas flow in pulse tubes has been investigated by numerical simulation. The geometry consists of a pulse tube and its flow straighteners. Viscous effects are taken into account, but heat exchange with the wall is not considered. Alternating boundary conditions are imposed which result in velocity fields with opposing directions. The net flow field is obtained by adding up these two velocity fields. In this way a new type of streaming has been found. In quasi-DC flow two loops are formed in the tube. With harmonic pressure oscillations the time-averaged velocity field also shows this new type of streaming. This paper analyzes this phenomenon and explains how the flow resistance of the straightener and the viscous interaction with the tube wall result in the streaming.