At nodes in stationary waves
In a longitudinal standing wave, such as in an organ pipe, there are fixed locations in the pipe where molecules oscillate back and forth at maximum amplitude, but there is no variation in pressure- the density remains constant; this is a pressure node.
At 1/4 wavelength away from this node there is a pressure antinode, where the density alternates from high to low, with corresponding alternations in pressure. At the center of this antinode there is no particle motion, but particles on either side alternately move toward and away from the centre