Prius (Gen 3) :: Shudder And Vibration Occurs Between 30 To 40 Mph When Throttle Is At Half Way Point

Picked up a used 10 prius iv with 31k and have put 1200 miles so far. One troubling thing I have noticed is this weird shudder/vibration that occurs between 30-40mph when the throttle is at the half way point (area between the light green and dark green bar) Vibration similar to that of driving over the rumble strip on the highway but not nearly as harsh and the noise would be similar to when you have one rear window open in the car and it creates the pulsating wind noise.

It doesn't do it all the time and mashing the throttle seems to alleviate the problem. It doesn't happen so much when cruising around town but it happens every time I get near the top of the incline in our office park and the car continues to lose speed until such time as I get back on the gas but it continues to do it until I turn into the parking lot. Occasionally it does it when coasting down a hill with the throttle right near the point of crossing from light green to dark green on the screen.